S. Calas

Densification kinetics and structural evolution during sintering of silica aerogel

A silica aerogel was sintered at 1050°C, and the effect of densification on the pore structure was determined by nitrogen sorption. Based on the initial pore size distribution (psd) of the aerogel, the kinetics of densification and the evolution of the psd were calculated using the theory of viscous sintering. If the psd is ignored, and the sintering behavior is predicted using the average pore size, then the agreement with measurements is not very good. However, when the actual psd is used in the calculation, then the predictions are excellent for density versus time, surface area and mean pore size versus density, and psd versus time. The quality of the fit depends on the constitutive laws describing the response of the porous material to stresses; the best results are obtained using the laws originally derived for the cylinder model. The power-laws that describe the density dependence of the elastic modulus of aerogels do not seem to apply to the viscosity; reasons for this dependence are discussed.

Textural properties of densified aerogels

Silica aerogels were subjected to isostatic pressure using a Hg porosimeter. Nitrogen adsorption experiments were performed to estimate specific surface area, porous volume and pore size distribution. In a same way, gas permeability measurements have been carried out. During the first stages of compression, the specific surface area seems to be constant. The difference between the total pore volume and that measured by the BJH method (using the desorption part of the isotherm) is less important in compressed aerogels than in sintered ones. Permeability decrease with densification is faster in partially compressed aerogels. This behavior is correlated to disappearance of largest pores and has been associated to the evolution of the pore size distribution which evolves toward smaller pore diameters.

Brillouin scattering study of sintered and compressed aerogels

Elastic properties of densified aerogels were investigated by Brillouin light scattering in silica aerogels densified by viscous sintering and isostatic compression. Sound velocity and acoustic attenuation characterize the evolution of the elastic properties vs. the bulk density for the two sets of samples. Elastic modulus increases strongly during sintering while the attenuation α decreases, which is coherent with a larger connectivity in the solid network. Viscous flow sintering creates new siloxane bonds, eliminates pores and as expected, the aerogel stiffens. Compressed aerogel has a completely different behavior. In the low pressure range, elastic modulus decreases and α rises. This is attributed to breakage of links between clusters during compression. Weakening of the aerogels is the consequence of large strain of the solid network. For higher pressure, the density increase is accompanied by stiffening, suggesting that condensation occurs more than link breakage.

Effect of oxidation treatment on the crack propagation rate of aerogels

Abstract Typical silica aerogels obtained by alcohol supercritical drying are hydrophobic. After a low temperature heat treatment below 400°C, they become hydrophilic. The temperature of this heat treatment may increase the network connectivity by establishing new siloxane bonds. On the microscopic scale, this process induces a very small shrinkage during which the silica backbone strengthens and approaches that of pure porous silica. Silica aerogels, whatever the nature of their surface, hydrophobic or hydrophilic, exhibit a stress corrosion effect as previously reported. A comparison of the subcritical crack growth rates of differently heat treated aerogels is given. The experimental investigations are carried out under a controlled moisture of 50% RH using the double cleavage drilled compression (DCDC) technique. The crack length is optically measured within the range 10−10– 10 −5 m s −1 . For a given stress intensity factor KI, we observe in the stress corrosion domain that hydrophobic aerogels display a very low crack rate. This rate increases with the amount of surface silanols. For higher oxidation temperatures, the number of surface silanols decreases inducing a lowering of the crack rate.

Mechanical Strength Evolution from Aerogels to Silica Glass

The mechanical strength of silica aerogels and partially densified silica aerogels are investigated. This material allows the study of properties over a great range of porosity (0–95%). The experiments are performed in tension using a bending test. The mechanical strength increases by a 4 orders of magnitude over the whole range of porosity. The results are analyzed using two geometrical models (spheres and cylinder models) which account for the mean porosity properties of the materials and allows the mechanical strength evolution from aerogels to silica glass to be modelled.

From Nanocomposite Aerogels to Glass Ceramics for Nuclear Wastes Containment

Nanocomposite aerogel is proposed as a host matrix for the synthesis of glass ceramics. The large porosity is used as a sponge to incorporate chemical species getting a two phases material. We describe the steps of the synthesis of glass ceramics for nuclear waste containment, from nanocomposite aerogels loaded with actinides surrogates (Ce and Nd). The glass synthesis is obtained without melting, by a control of several solid phase transformations: sintering, viscous flow, crystallization and foaming. Thanks to their high resistance to thermal shock and water corrosion, these glass ceramics are certainly good candidates as actinides containment materials.