J. Phalippou

Synthesis of glasses from gels: the problem of monolithic gels

The synthesis of glasses from gels has recently aroused considerable interest both from the theoretical and practical standpoint. The conditions of gel formation which determine the structure of the wet gel are first examined. During the drying process, which is a decisive step in the obtention of monolithic gels, an essential role is played by capillary forces. It is shown that, among the various methods of obtaining monolithic gels, that based on the hypercritical evacuation of the solvent is the most reliable.

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.

Crystallization of gels and glasses made from hot-pressed gels

Abstract Silica gels were prepared by two different methods: (1) destabilization of a silica hydrosol (gel 1); (2) hydrolysis and polycondensation of a tetra-methoxysilane (gel 2). The crystallization of the gels was then studied as a function of the temperature by means of X-ray diffraction. It was evident that the crystallization was strongly influenced by the amount of alkali oxides present in the gel. It is effectively the concentration of akali impurities which could explain the lower temperature of crystallization necessary for gel 1 compared with that of gel 2. During the crystallization of the gel containing Na2O the crystalline phase of silica which appears first is the cristobalite; with Li2O it is quartz. The effect of additives such as boric anhydride was studied. This oxide was found to reduce the tendency of the gels to crystallize. The glasses of the system SiO2B2O3 obtained by the hot-pressing of the gels confirmed this phenomenon. Above 10 mol% B2O3 it was impossible to crystallize the gels and the glasses of this system under two hours.

Synthesis of monolithic silica gels by hypercritical solvent evacuation

The method of obtaining monolithic dry gels by hypercritical solvent evacuation is presented. The influence of various parameters on the possibility of obtaining crack-free pieces is studied in some detail. The influence of these parameters on the final properties of the gel is also discussed.

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.

Raman study of alumina gels

The sol-gel transition of an alumina sol is followed by Raman spectroscopy. Other spectroscopy techniques are also used to obtain additional information. The sol is made of particles of pseudo-boehmite which give rise to a Raman line at 360 cm−1. The treatment of the sol by ethyl-ether extraction of 2-butanol confirms the presence of this band. Its assignment is made after a study of the spectra obtained during gelation on wet and dried gels. Raman spectroscopy does not show the presence of aluminium polycations in the sol. However, 27Al nuclear magnetic resonance shows clearly the peak of the tetrahedrally coordinated aluminium atom located at the centre of these polycations. The presence of polycations contributes to explaining the slope change observed in the curve giving the Raman line intensities as a function of the aluminium content of the sample. X-ray diffraction and infrared spectroscopy enable us to correlate the Raman spectra of the gels heat-treated at various temperatures with respective structures.

Glasses from aerogels

Silica glasses are obtained by the densification of aerogels. The transformation of the material into a glass is followed by differential thermal analyis, thermo-gravimetric analysis, dilatometry and by the evolution of the structural, textural and mechanical properties of the material. The organic species and the hydroxyl groups are removed by oxidation and chlorination heat treatments in such a way as to avoid bloating and crystallization phenomena during sintering. Densification is obtained by heat treatment at a low temperature (1100 to 1300 ° C). The densified aerogel shows physical properties identical to those of molten silica. Moreover, this material is very pure and its water content is low. The same process can be extrapolated to multicomponent glasses and composite materials.

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.