Plasticity in aerogels

Abstract

Abstract. When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature.The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material.The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.

Analysis of mercury porosimetry curves of precipitated silica, as an example of compressible porous solids

“…The elastic bulk modulus at any point along the compression curve (P vs. V Hg ) is defined by the relation dP = −K·(dV/V). This mechanical property, obtained using the volume of the sample [7,8] as V, agrees well with the modulus determined by beambending [7] or sound velocity techniques [9]. b) Structure deformation.…”

“…Pirard et al [11] proposed that the largest pores between the clusters collapse as a result of pore edge collapse and their scraps form smaller pores. In this process the internal cluster structure does not change [9] but, as the clusters interpenetrate each other modifying their periphery, the network connectivity increases [10]. To sum up, the compression results in better packing of clusters.…”

Producing Crack-Free Colloid−Polymer Hybrid Gels by Tailoring Porosity

“…In the case of aerogels: 30,31 the elastic deformation is the result of the contacts generated between solid parts, which where not initially in contact, while plastic strain is explained by the restructuring of the solid network. It seems proven that this irreversible modification is the consequence of the formation of siloxane bonds due to reactions of condensation between silanols, which still exist in TEOS aerogel.…”

Aerogel Sintering: From Optical Glasses to Nuclear Waste Containment

“…For low-pressure values, the strain due to external pressure returns to zero value with pressure release. This is a quite interesting experiment allowing to determine the bulk compression modulus of the aerogel (Woignier et al 1997).…”