STRESS DEVELOPMENT DURING SUPERCRITICAL DRYING

Although supercritical drying eliminates the capillary stresses that cause damage during conventional drying, there are other sources of stress and strain that can destroy aerogels. During heating in the autoclave, expansion of the pore liquid causes dilatation of the gel network. The effect depends on the heating rate, and can cause cracking if the rate is excessive. Syneresis is also accelerated during heating, and can cause considerable differential shrinkage and stress. The most severe stresses occur when the gel is in contact with its container, so that radial expansion and radial flow of liquid are prevented. Methods of calculating these stresses are discussed, and the magnitudes of the stresses are estimated. The results are shown to be consistent with experimental observations.