Mechanical properties of silica aerogels

Aerogels are extremely low density solids that are characterized by a high porosity and pore sizes in the order of nanometers. The mechanical behavior of silica aerogel was investigated with hardness, compression, tension and shear tests. The influences of testing conditions, storage environment and age were examined, with particular attention paid to the effects of processing parameters, including fiber-reinforcement. Good correlation was found between hardness and compressive strength over a wide range of processing parameters. Increasing fiber reinforcement generally retarded shrinkage during fabrication and yielded smaller matrix densities for a given target density. For a given fiber content, hardness, compressive strength and elastic moduli increased and strain at fracture decreased with increasing matrix density. In the lower ranges of matrix density, fiber reinforcement increased strain at fracture and elastic moduli. The mechanical response was also sensitive to environment and storage history. With age, the compressive strength and elastic moduli increased while the strain at fracture decreased. Tension and shear results indicate that shear strength of aerogels exceeds tensile strength which is consistent with brittle materials response. (C) 1998 Elsevier Science B.V.