Thermal conductivity/structure correlations in thermal super-insulating pectin aerogels.

Pectin aerogels were synthesized via dissolution-solvent exchange-drying with supercritical CO2. The goal was to correlate thermal conductivity with aerogel morphology and properties in order to understand how to obtain a thermal super-insulating material with the lowest possible conductivity. Polymer concentration, solution pH and presence of bivalent ions were varied to tune pectin gelation mechanism and the state of matter, solution or gel. For the first time for bio-aerogels, a U-shape curve of thermal conductivity as a function of aerogel density was obtained. It shows that to reach the lowest conductivity values, a compromise between density and pore sizes is needed to optimize the inputs from the conduction of solid and gaseous phases. The lowest value of conductivity, 0.015 W/m K, was for aerogels from non-gelled pectin solutions. Calcium-induced gelation leads to pectin aerogels with very low density, around 0.05 g/cm3, but with many macropores, thus reducing the contribution of Knudsen effect.