Florence Despetis

Mechanical Properties of Gel-Derived Materials

The mechanical behaviour of xerogels and aerogels is generally described in terms of brittle and elastic materials, like glasses or ceramics. The main difference compared to silica glass is the order of magnitude of the elastic and rupture moduli which are 104 times lower. However, if this analogy is pertinent when gels are under a tension stress (bending test) they exhibit a more complicated response when the structure is submitted to a compressive stress. The network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. As a consequence of the plastic shrinkage it is possible to densify and stiffen the gel at room temperature. These opposite behaviours (elastic and plastic) are surprisingly related to the same two kinds of gel features: the silanol content and the pore volume. Both elastic modulus and plastic shrinkage depend strongly on the volume fraction of pores and on the condensation reaction between silanols. On the mechanical point of view (rupture modulus and toughness), it is shown that pores and silanols play also an important role. Pores can be considered as flaws in the terms of fracture mechanics and the flaw size, calculated from rupture strength and toughness is related to the pore size distribution. Different kinds of gels structure (fractal or not fractal) have been synthesized by a control of the different steps of transformation such as sintering and plastic compaction. The relationships between structural and the elastic properties are discussed in terms of the percolation theory and fractal structure.

Experimental Investigations on the Thermal Conductivity of Silica Aerogels by a Guarded Thin-Film-Heater Method

A thin-film-heater method is setup to measure the thermal conductivity of super insulating materials such as silica aerogels. The experimental setup is purposely designed for insulating materials and allows direct measurement of the thermal conductivity. Few experimental data are available in the literature concerning thermal conductivity of aerogels even though these materials are of major interest in thermal insulation. More data are necessary in order to understand thermal transport and to validate existing models. Monolithic and granular silica aerogels are investigated. Our experimental technique enables to quantify the influence of important parameters, such as air pressure and distribution of grain sizes, on the insulating performance of this material.

Very large-scale structures in sintered silica aerogels as evidenced by atomic force microscopy and ultra-small angle X-ray scattering experiments

During the last few years the bulk structure of silica aerogels has been extensively studied mainly by scattering techniques (neutrons, X-rays, light). It has been shown that small silica particles aggregate to constitute a fractal network. Its spatial extension and fractal dimension are strongly dependent on the synthesis conditions (e.g., pH of gelifying solutions). These typical lengths range from 1 to 10 nm. Ultra-small angle X-ray scattering (USAXS) and atomic force microscopy (AFM) experiments have been carried out on aerogels at different steps of densification. The results presented in this paper reveal the existence of a spatial arrangement of the solid part at a very large length scale. The evolution of this very large-scale structure during the densification process has been studied and reveals a contraction of this macro-structure made of aggregates of clusters.

Evidence for a disorder-driven phase transition in the condensation of 4He in aerogels

We report on thermodynamic and optical measurements of the condensation process of

Hydrophobic silica CO2 aerogels

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Effect of oxidation treatment on the crack propagation rate of aerogels

He in three silica aerogels of different microstructures. For the two base-catalysed aerogels, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder driven transition, in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalysed aerogel, which we interpret as due to a larger disorder in this case.

Measurements of Helium Density in Aerogel near the Liquid-Vapor Critical Point

A hydrophobic silica gel is prepared by hydrolysis of a triethoxysilane solution. The SiH group induces a repelling water effect. CO2 hypercritical drying is performed on such a gel. The obtained aerogel exhibits hydrophobic properties similar to those of the starting gel. Thermal properties are investigated and show that the SiH group is not stable at temperatures > 300°C. Consequently the aerogel gains weight and rapidly sinters in this temperature range.

Subcritical crack velocity in silica 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

We investigate the behavior of 4He confined in silica aerogels near the bulk liquid-vapor critical point. Using a new mechanical technique to measure the density of the confined 4He along isotherms, we find that the density continuously increases from a low density phase up to a dense phase as the pressure is increased up to slightly below the bulk saturated pressure. An hysteretic behavior is observed between emptying and filling, which is not uniquely due to thermal problems. We argue that, our observations are more in favor of some kind of capillary condensation than of a genuine first order phase transition.

Condensation of helium in aerogel and athermal dynamics of the random-field Ising model.

Abstract Crack velocity in aerogels was measured using the double cleavage drilled compression technique. Crack length was followed using an optical equipment which allows measurement velocity from 10 −10 m/s to 10 −2 m/s. Stress intensity factor, K I , was calculated using a polynomial relationship which expresses K I as a function of stress and crack length. Crack velocity evolves as a function of the stress intensity factor and shows that silica aerogels exhibit a stress corrosion effect. The velocity curve depends on the structure of the aerogel at the moment of the experiment. For a given K I , curves were shifted to higher velocities when aerogel was stressed under atmosphere moisture. The different behaviours are explained on the microstructure scale according to previous molecular orbital computations of the silica structure involving rings opening. The aerogel which is macroscopically originally hydrophobic does not totally impede water molecules from reaching the crack tip. Stress corrosion is estimated from the chemical susceptibility factor, n , whose value evolves from 35 (like silica glass) to 13.