F. Despetis - Academia.edu (original) (raw)

Papers by F. Despetis

Research paper thumbnail of Effect of Aging on Mechanical Properties of Resorcinol-Formaldehyde Gels

Resorcinol-Formaldehyde gels have been prepared in aqueous solutions. After a gelification stage ... more Resorcinol-Formaldehyde gels have been prepared in aqueous solutions. After a gelification stage at 80 • C, an aging was performed in water or acetic acid solutions at ambient temperature or in the parent liquid at 80 • C for different durations. Shear modulus of gels immersed in water is measured using the 3 points bending technique. The evolution of elastic constant with time depends on the pH of aging solution. The strengthening is more pronounced for aging under acidic conditions than in water. Nevertheless when the aging stage is performed at 80 • C for a few days, quite identical values of shear modulus are obtained comparatively to acid treatment. The enhancement of mechanical properties of RF gels in acidic conditions is related to polycondensation reaction rates.

Research paper thumbnail of Section 1. Slow crack growth

Research paper thumbnail of Comparison between sintered and compressed aerogels

Optical Materials, 2004

Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compre... more Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compression. We report here a comparative analysis of silica aerogel densified by these two methods. To better follow their structural evolutions we use SAXS measurements performed on aerogels exhibiting a fractal geometry. This fractal geometry specially gives information about the way the solid network is firstly established and how it evolves with densification. The structural features such as particle and cluster sizes are observed to change differently according to the densification method. While the specific surface area of sintered aerogels decreases with densification, it does not change when densification is performed under isostatic compression. Furthermore the pore size distribution analysis evidences that the pressure induces the collapse of the largest pores while sintering acts on all pores. A microscopic model is proposed. It allows to explain the structural changes observed both by isostatic compression and by thermal sintering. Moreover, it agrees well with the evolution of elastic constant and internal friction with densification.

Research paper thumbnail of Ultralow density silica aerogels by alcohol supercritical drying

Journal of Non-Crystalline Solids, 1998

Synthesis of ultralow density aerogels requires full transformation of diluted monomers into soli... more Synthesis of ultralow density aerogels requires full transformation of diluted monomers into solid. Shrinkage during ageing and supercritical drying must be controlled as low as possible. Reported investigations deal with achievement in the optimisation of the two-step process parameters performed using classical acid and basic steps. Acid step involves the formation of partially hydrolysed and partially condensed silica mixture in ethanol. Different compositions of tetraethoxysi-Ž . lane, ethanol and hydrochloric acid TEOS:ETOH:HCl have been prepared to provide suitable gelling time and to enhance mechanical properties of final gels. Supercritical drying was performed using modified solutions mixtures which prevent silica dissolution. Under these conditions gel texture is preserved as demonstrated by the low shrinkage. This route permits positioning of uncracked transparent aerogels with densities lower than 3 kgrm 3 . Specific surface area values are in the range of usual aerogels. Therefore, because of their unusual large pores, specific surface area decreases with density. q 1998 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Crack path instabilities in DCDC experiments in the low speed regime

Journal of Non-Crystalline Solids, 2003

Research paper thumbnail of Subcritical crack growth in silica aerogel

Journal of Non-Crystalline Solids, 2004

ABSTRACT The sub-critical crack growth domain in hydrophilic silica aerogel was studied using the... more ABSTRACT The sub-critical crack growth domain in hydrophilic silica aerogel was studied using the Double-Cleavage-Drilled-Compression test (DCDC). The effect of temperature and water vapor content on the crack growth rate is measured. The experiments are carried out between 295 and 383 K with relative air moisture ranging from 5% to 80% Rh. Like dense silica, crack growth velocities were found to decrease when increasing the temperature at a constant water vapor concentration. Surprisingly, the crack velocity also decreases when increasing water vapor content at constant temperature. Such a behavior is contrary to expected results usually encountered in glasses. An analogy with nitrogen adsorption experiments on the same aerogels is used to explain this behavior. We assume that a capillary condensation phenomenon of water vapor inside the aerogel porosity takes place, inducing internal compressive stresses.

Research paper thumbnail of Hydrophobic silica CO2 aerogels

Journal of Non-Crystalline Solids, 1993

ABSTRACT

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

Journal of Non-Crystalline Solids, 2001

During the last few years the bulk structure of silica aerogels has been extensively studied main... more 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 dierent steps of densi®cation. 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 densi®cation process has been studied and reveals a contraction of this macro-structure made of aggregates of clusters. Ó

Research paper thumbnail of Effect of Aging on Mechanical Properties of Resorcinol-Formaldehyde Gels

Resorcinol-Formaldehyde gels have been prepared in aqueous solutions. After a gelification stage ... more Resorcinol-Formaldehyde gels have been prepared in aqueous solutions. After a gelification stage at 80 • C, an aging was performed in water or acetic acid solutions at ambient temperature or in the parent liquid at 80 • C for different durations. Shear modulus of gels immersed in water is measured using the 3 points bending technique. The evolution of elastic constant with time depends on the pH of aging solution. The strengthening is more pronounced for aging under acidic conditions than in water. Nevertheless when the aging stage is performed at 80 • C for a few days, quite identical values of shear modulus are obtained comparatively to acid treatment. The enhancement of mechanical properties of RF gels in acidic conditions is related to polycondensation reaction rates.

Research paper thumbnail of Section 1. Slow crack growth

Research paper thumbnail of Comparison between sintered and compressed aerogels

Optical Materials, 2004

Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compre... more Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compression. We report here a comparative analysis of silica aerogel densified by these two methods. To better follow their structural evolutions we use SAXS measurements performed on aerogels exhibiting a fractal geometry. This fractal geometry specially gives information about the way the solid network is firstly established and how it evolves with densification. The structural features such as particle and cluster sizes are observed to change differently according to the densification method. While the specific surface area of sintered aerogels decreases with densification, it does not change when densification is performed under isostatic compression. Furthermore the pore size distribution analysis evidences that the pressure induces the collapse of the largest pores while sintering acts on all pores. A microscopic model is proposed. It allows to explain the structural changes observed both by isostatic compression and by thermal sintering. Moreover, it agrees well with the evolution of elastic constant and internal friction with densification.

Research paper thumbnail of Ultralow density silica aerogels by alcohol supercritical drying

Journal of Non-Crystalline Solids, 1998

Synthesis of ultralow density aerogels requires full transformation of diluted monomers into soli... more Synthesis of ultralow density aerogels requires full transformation of diluted monomers into solid. Shrinkage during ageing and supercritical drying must be controlled as low as possible. Reported investigations deal with achievement in the optimisation of the two-step process parameters performed using classical acid and basic steps. Acid step involves the formation of partially hydrolysed and partially condensed silica mixture in ethanol. Different compositions of tetraethoxysi-Ž . lane, ethanol and hydrochloric acid TEOS:ETOH:HCl have been prepared to provide suitable gelling time and to enhance mechanical properties of final gels. Supercritical drying was performed using modified solutions mixtures which prevent silica dissolution. Under these conditions gel texture is preserved as demonstrated by the low shrinkage. This route permits positioning of uncracked transparent aerogels with densities lower than 3 kgrm 3 . Specific surface area values are in the range of usual aerogels. Therefore, because of their unusual large pores, specific surface area decreases with density. q 1998 Elsevier Science B.V. All rights reserved.

Research paper thumbnail of Crack path instabilities in DCDC experiments in the low speed regime

Journal of Non-Crystalline Solids, 2003

Research paper thumbnail of Subcritical crack growth in silica aerogel

Journal of Non-Crystalline Solids, 2004

ABSTRACT The sub-critical crack growth domain in hydrophilic silica aerogel was studied using the... more ABSTRACT The sub-critical crack growth domain in hydrophilic silica aerogel was studied using the Double-Cleavage-Drilled-Compression test (DCDC). The effect of temperature and water vapor content on the crack growth rate is measured. The experiments are carried out between 295 and 383 K with relative air moisture ranging from 5% to 80% Rh. Like dense silica, crack growth velocities were found to decrease when increasing the temperature at a constant water vapor concentration. Surprisingly, the crack velocity also decreases when increasing water vapor content at constant temperature. Such a behavior is contrary to expected results usually encountered in glasses. An analogy with nitrogen adsorption experiments on the same aerogels is used to explain this behavior. We assume that a capillary condensation phenomenon of water vapor inside the aerogel porosity takes place, inducing internal compressive stresses.

Research paper thumbnail of Hydrophobic silica CO2 aerogels

Journal of Non-Crystalline Solids, 1993

ABSTRACT

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

Journal of Non-Crystalline Solids, 2001

During the last few years the bulk structure of silica aerogels has been extensively studied main... more 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 dierent steps of densi®cation. 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 densi®cation process has been studied and reveals a contraction of this macro-structure made of aggregates of clusters. Ó