Search

A thorough understanding of the mechanisms and factors involved in the dynamics of organic carbon in soils is required to identify and enhance natural sinks for greenhouse gases. Some tropical soils, such as Andosols, have 3–6 fold higher concentrations of organic carbon than other kinds of soils containing classical clays. In the tropics, toxic pesticides permanently pollute soils and contaminate crops, water resources, and ecosystems. However, not all soils are equal in terms of pesticide contamination or in their ability to transfer pollution to the ecosystem. Andosols are generally more polluted than the other kinds of soils but, surprisingly, they retain and trap more pesticides, thereby reducing the transfer of pesticides to ecosystems, water resources, and crops. Andosols thus have interesting environmental properties in terms of soil carbon sequestration and pesticide retention. Andosols contain a nano porous clay (allophane) with unique structures and physical properties compared to more common clays; these are large pore volume, specific surface area, and a tortuous and fractal porous arrangement. The purpose of this mini review is to discuss the importance of the allophane fractal microstructure for carbon sequestration and pesticide trapping in the soil. We suggest that the tortuous microstructure (which resembles a labyrinths) of allophane aggregates and the associated low accessibility partly explain the poor availability of soil organic matter and of any pesticides trapped in andosols.

Mesoporous sol-gel glasses may play the role of host materials to study the effect of confinement on fluid properties. A method based on a peculiar drying procedure of silica gels, followed by their partial densification by a heat treatment is described. Monolithic mesoporous glasses with a porosity exceeding 50%, a well defined pore size and a controlled surface roughness are obtained. N2adsorption-desorption and Small Angle X-ray Scattering techniques are used to study the porous and solid texture, and the inner surface roughness of these materials. Various samples of narrow pore size distribution with half width at half height under 2nm, and mean pore size below 25nm are obtained.

A thorough understanding of the mechanisms and factors involved in the dynamics of organic carbon in soils is required to identify and enhance natural sinks for greenhouse gases. Some tropical soils, such as Andosols, have 3–6 fold higher concentrations of organic carbon than other kinds of soils containing classical clays. In the tropics, toxic pesticides permanently pollute soils and contaminate crops, water resources, and ecosystems. However, not all soils are equal in terms of pesticide contamination or in their ability to transfer pollution to the ecosystem. Andosols are generally more polluted than the other kinds of soils but, surprisingly, they retain and trap more pesticides, thereby reducing the transfer of pesticides to ecosystems, water resources, and crops. Andosols thus have interesting environmental properties in terms of soil carbon sequestration and pesticide retention. Andosols contain a nano porous clay (allophane) with unique structures and physical properties compared to more common clays; these are large pore volume, specific surface area, and a tortuous and fractal porous arrangement. The purpose of this mini review is to discuss the importance of the allophane fractal microstructure for carbon sequestration and pesticide trapping in the soil. We suggest that the tortuous microstructure (which resembles a labyrinths) of allophane aggregates and the associated low accessibility partly explain the poor availability of soil organic matter and of any pesticides trapped in andosols.

The evolution of the texture of a silica aerogel during sintering is studied by thermoporometry. During the first stages of the densification the macroporous volume drops strongly while the mesoporous volume seems to be constant. This behavior is in contradiction with sintering models which predict that the sintering rate is faster for bodies with smaller particles or pores. In fact the analysis of the micropore size distribution shows that the sintering of the mesopores is responsible for macropore decrease.

Allophanic soils behave as gels during a classical drying with an important irreversible shrinkage which can modify the soil physical properties. These soils are interesting in terms of mitigation of the greenhouse effect (C sequestration) and the knowledge of the true soils features are of importance. Like for silica gels, we use the CO2supercritical drying procedure (SCD) to control the drying step. The objective is to preserve the structural and textural properties of the soils, which can be affected by the classical drying. We show also that despite the necessary solvent exchange and high pressure of the supercritical drying, the chemical composition and the allophanic features are not strongly affected. The textural properties such as specific surface area are higher for the supercritical dried samples compared to the classical dried samples, indicating the preserving effect of the SCD and showing the interest of the application of this drying method to this natural and complex “gels”. With these data, we show possible effects of the specific surface area on the C and N content of the allophanic soils.

Abstract The pore size distribution in silica gels can be tailored by the addition of silica soot particles during the gel formation. We introduce a numerical model in order to simulate the structure of this “composite gel”. The algorithm is based on Diffusion-Limited Cluster-Cluster Aggregation model with an initial binary distribution of monomer sizes. The textural properties of the simulated gels are calculated using a simple triangulation method. Nitrogen adsorption-desorption experiments show that with the powder addition the mean pore size is shifted towards larger pore size and the specific surface area decreases. Numerical results of the mean pore size, specific surface area, and particles are in good agreement with experimental data. Because of these textural properties this new type of gels and aerogels has larger permeability and interesting properties as host matrix. The composite gels and the numerical model could also be helpful to simulate the natural allophanic gel found in volcanic soils.

Silica gels (classical aerogels and composite aerogels) have been prepared by classical gelation and addition of silica soot in the gelifying solution before gelation. Due to the aggregation mechanisms, these structures are characterized by a fractal organization. The fractal network previously described in the literature (1–100 nm) which results from the aggregation mechanism of the organosiloxane is affected by the addition of the silica soots. Ultra Small Angle X-ray Scattering (USAXS) experiments (done at ESRF) shows that besides the fractal network built by the organosiloxane, the silica soots are forming another porous structure at a higher scale. The mechanical properties seem to be dependent on this large pore structure. Under isostatic pressure, aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, by the pore collapse tending towards the silica glass. The densification mechanism is different from the one obtained by a sintering at high temperature. The pore collapse mechanism is favored by the large pores structure of the composite aerogels, in contrast to viscous sintering.

Abstract For long life nuclear wastes (essentially actinides) research is in progress to propose new matrices with an improvement of the chemical durability. High silica content glasses present high chemical durability, and we describe a process to prepare silica glass embedding the nuclear waste. Porous silica (gel) is used as a host matrix for nuclear waste. Neodymium oxide and cerium oxide are used to simulate the actinide oxides. The gel is soaked in a solution containing the simulant in nitrate salt form.

Abstract 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.

Under isostatic pressure aerogels display an irreversible shrinkage caused by plastic deformation. As a consequence of this plastic shrinkage it is possible to densify, and thus to modify the elastic properties of aerogels. We show that the elasticity of the material is strongly influenced by structural transformation, and we observe a weakening of the network during the first stage of densification. The structural evolution, followed by Small Angle X-ray Scattering, shows that the densification mechanism is different from the one obtained by a sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, in contrast to viscous sintering.

When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature. The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material. The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.

The mechanical behaviour of silica aerogels in alcoholic environment has been interpreted in terms of stress corrosion in analogy with silica glass. The chemical susceptibility factor has been determined by the dynamical method and we also measured the Weibull's modulus which characterizes the strength distribution. These data show a stress corrosion effect which is significant in alcoholic atmosphere. The results could explain a possible fracture of gels during the supercritical drying treatment as already observed.

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.

Experiments carried out to obtain monolithic silica aerogels are reported. The different ways to synthesize silica aerogels from alkoxides are investigated. The manner by which the critical point of the solvent can be reached is discussed as a function of the nature of the solvent. Physical parameters such as the amount of additional solvent or pressure, play a very important role in the direction of the liquid-vapour interface displacement during the heating of the autoclave. Some experiments performed using hypercritical drying with CO2 liquid are also mentioned.

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.

The evolution of the texture of silica aerogels during sintering is studied by thermoporometry for both neutral and base catalysed materials.

Density measurements of acid-catalysed aerogels have been carried out using helium pycnometry technique. The density increases from 1.85 to 2 when the volume percentage of tetramethoxysilane of the starting solution increases from 6 to 33. The aerogels submitted to an oxidation heat treatment show higher density values varying between 2 and 2.2. The variation of the skeletal density with the volume percentage of tetramethoxysilane is explained by a simple model taking into account the size of the primary particles which build the gel network. The observed difference between the measured density and the bulk density of amorphous silica (2.2) is attributed to the presence of organic residues located on the particles surface. The increase of skeletal density with the oxidation treatment is proposed to be due to the smoothing of the particle surface or to an increase of the particle size occurring during the oxidation treatment.

The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups.

Since gels are typically brittle materials, reliable analyses are essential to determine the strength distribution. The mechanical strength of silica gels has been measured by the three point bending technique. Based on the Weibull function, the statistical analysis allows determination of the Weibull modulus (m), which characterizes the strength distribution and the flaw size effect.

When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature. The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material. The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.

The mechanical behaviour of silica aerogels in alcoholic environment has been interpreted in terms of stress corrosion in analogy with silica glass. The chemical susceptibility factor has been determined by the dynamical method and we also measured the Weibull’s modulus which characterizes the strength distribution. These data show a stress corrosion effect which is significant in alcoholic atmosphere. The results could explain a possible fracture of gels during the supercritical drying treatment as already observed.

The fractal character of silica aerogels has been undoubtedly established by small-angle neutron scattering experiments (SANS). These materials are particularly well suited to the study of the vibrational dynamics of fractal media. Brillouin scattering spectra have given strong evidence for a cross-over from long-wavelength phonons to 'fractons' in the gigahertz range. The main quantities deduced from these experimental results, namely the non-dispersed velocity of sound and the phonon-fracton cross-over frequency, are found to scale with the density of the materials. From the scaling exponents, values of the fractal and spectral dimensions are derived. The fractal dimension is very close to that deduced from SANS data. The results of a preliminary study of vibrational modes in the terahertz range by Raman scattering are also presented and briefly discussed.

The density of vibrational states in a fractal silica aerogel of Hausdorff dimensionD?2.4 has been determined using spin-echo neutron spectroscopy. A smooth phonon-fracton crossover is found with a fracton spectral dimension of $$\overline{\overline d} \simeq 1.4$$ . This direct measurement of $$\overline{\overline d} $$ near crossover agrees with a previous finding based on scaling Brillouin-spectroscopy results obtained with several samples. We also show that protons adsorbed in the gel vibrate with substantially higher amplitude than the silica matrix.