T. Woignier

Different kinds of structure in aerogels: relationships with the mechanical properties

Abstract The density and the structure (fractal and non-fractal) of aerogels are modified either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process or by an isostatic pressure deformation. These aerogels have porosities ranging from 98% to 0%. The mechanical properties of the different aerogels (elastic modulus and strength) measured by 3 point bending, are dependent on their structure; they vary by five orders of magnitude as a function of density and follow power law evolution. However for the same relative density the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. These structural differences have been observed by SAXS experiments. The effects of the sintering process compared to that of the plastic transformation on the mechanical properties are explained by the associated structural changes. Sintering increases the network connectivity and the densification by compression leads to a new spatial arrangement of the clusters but their internal structure is not affected. In addition, relationships between structural and porous features and the mechanical properties are discussed in terms of percolation theory and the fractal approach. We show that the exponent of the power law does not depend on the fractal feature and percolation is only an approximation which cannot describe results.

Mechanical strength of silica aerogels

Abstract Pure silica aerogels are obtained by hypercritical evacuation of the solvent. The strength is measured by the three-point flexural test on monolithic parallelepipedic samples and by a diametral compression test on cylindrical samples. The stress-strain curve shows a perfect elastic behaviour and the “conchoidal” fracture morphology indicates that the material is as brittle as a conventional glass. The mechanical properties are followed as a function of the bulk density. Aerogels with the highest porosity ( P > 95%) reveal a maximum flexural strength lower than 10 −2 MPa. A model is proposed to account for the obtained mechanical properties.

Acoustic properties and potential applications of silica aerogels

Abstract Acoustic properties of cylindrical silica aerogels in both ultrasonic and audible range are presented. Velocity measurements for low ultrasonic frequencies show that the low-density aerogels can exhibit unexpected attenuation for well-defined frequency bands. Measurements of the acoustical impedance of samples in the audible range show that the results depend dramatically on the geometry and/or the boundary conditions imposed to the samples. The ‘attenuation’ bands in which the samples present an unexpected high attenuation are related to the aerogel density. These particular results are discussed in two ways; first for application purposes and second in terms of a possible theoretical explanation. Neither the classical theory of propagation in a homogeneous material nor the Biot theory for porous materials can explain the results.

Polyurethane based organic aerogels and their transformation into carbon aerogels

Abstract New organic gels were prepared from chemical reactions conventionally used to make polyurethane foams. Reactions were carried out using CH 2 Cl 2 as solvent. Solvent exchange occurs directly in the autoclave by flushing the gel with supercritical CO 2 . The subsequent organic aerogels were obtained by a classical CO 2 supercritical drying process. They are non-transparent. The thermal evolution to a carbon aerogel was investigated with a starting polymer aerogel having a bulk density of 0.24 g/cm 3 and a specific surface area of 300 m 2 /g. As the temperature increases the specific surface area and the bulk density increase for temperatures higher than 400°C. The pore morphology strongly depends on the temperature as evidenced by scanning electron microscopy experiments. The pyrolyzed aerogel has the texture of an ultrafine celled foam. Thermogravimetric analysis was related to dilatometric measurements and the aerogel density versus temperature was estimated. Carbon aerogels were obtained at temperatures of 600–800°C.

Monolithic aerogels in the systems SiO2B2O3, SiO2P2O5, SiO2B2O3P2O5

Abstract The gels in the binary and ternary systems: SiO 2 B 2 O 3 , SiO 2 P 2 O 5 , SiO 2 B 2 O 3 P 2 O 5 ; were prepared by hydrolysis and polydensation of metalorganic compounds. The gelling times vary with the molar percentage of SiO 2 . The solvent was evacuated under hypercritical conditions in an autoclave in order to obtain aerogels free of cracks. The monolithicity of the aerogels is influenced by the method of preparation of the alcogels. The crystallization of BPO 4 was observed in the ternary system only. These materials can be converted into glasses by heat treatment. The structural evolution was followed by means of infrared spectroscopy and textural evolution by dilatometric measurements.

Effect of aging and pH on the modulus of aerogels

Abstract The elastic modulus E of an aerogel is related to its density ϱ by E = c(pH)ϱ3.7 for gels made at various pH values; however, the constant of proportionality c depends on pH. It is found that this relation applies regardless of whether the density is varied by lengthy aging or by increasing the concentration of tetramethoxysilane (TMOS) in the initial sol; that is, changes in aging time and [TMOS] evidently have equivalent effects on gel structure. The influence of pH on densification can be understood in terms of competition between polycondensation (P, which produces shrinkage) and dissolution/ reprecipitation (D/R, which increases the rigidity of the network). At low pH, P dominates, but at high pH D/R stiffens the network and inhibits shrinkage.

Parameters affecting elastic properties of silica aerogels

The Youngs modulus of silica aerogels is measured, using the three point flexural technique. Various parameters have been investigated, such as the concentration of silicon compounds as well as the catalyting conditions used to develop the initial alcogel and the heat treatment performed to densify the material. The elastic behavior of aerogels depends on the conditions of gel preparation, and the Youngs modulus is shown to obey the power dependence E ≃ ρ 3.7 . This scaling exponent is compared to the percolation model predictions. On the other hand, the elastic behavior of different sets of aerogels is related to the spatial arrangement of the particles pointed out by fractal features.

Plastic behaviour of aerogels under isostatic pressure

The bulk modulus of a set of aerogels was measured as a function of applied pressure by means of a Hg porosimeter. With increasing pressure, the aerogels, which are known to be elastic, display irreversible shrinkage corresponding to plastic behaviour. The magnitude of the plastic shrinkage and the value of the bulk modulus depend strongly on the volume fraction of solid in the aerogel. The plastic behaviour is related to the condensation reaction between silanols. Both plastic shrinkage and bulk modulus are clearly increased when the aerogel has undergone an oxidation treatment which induces the formation of silanols at the expense of organic groups. As a consequence of this plastic shrinkage, it is possible to densify and stiffen the aerogel at room temperature. The connectivity of the aerogel is strongly increased by the densification under isostatic pressure and the power laws describing the elastic constant evolution as a function of the bulk density lead to values of the power exponent different to those found in the literature. Fatigue effect due to cycling runs of pressurization and also delayed elasticity have been observed for the stiffer aerogels.

Skeletal density of silica aerogels

The skeletal density of silica aerogels, produced by hypercritical drying of gels, is studied by helium pycnometry. The bulk and the skeletal densities vary as a function of parameters such as TMOS concentration. pH and densifying heat treatment. Skeletal densities were found to be slightly lower than that of vitreous silica. The results are compared to values obtained on xerogels.

Biot's theory of acoustic propagation in porous media applied to aerogels and alcogels

Comparison of acoustic propagation in alcogels and aerogels presents an interesting difference for the high porosity: in alcogels, longitudinal wave velocity remains around the velocity in alcohol while, in aerogels, it is significantly lower than the velocity in air. In this paper, Biots model of acoustic propagation in porous media is applied. We find a good agreement with measurements in terms of velocity that validates this theory for both alcogel and aerogel cases. Qualitative agreement is obtained for absorption but further comparisons are needed to test quantitative agreement. The theory allows explanation of behaviour differences observed by distinguishing the influence of the solid and fluid phases from that of the gel network.