Deformation of Silica Aerogel During Fluid Adsorption

Aerogels are very compliant materials - even small stresses can lead to large deformations. In this paper we present measurements of the linear deformation of high porosity aerogels during adsorption of low surface tension fluids, performed using a Linear Variable Differential Transformer (LVDT). We show that the degree of deformation of the aerogel during capillary condensation scales with the surface tension, and extract the bulk modulus of the gel from the data. Furthermore we suggest limits on safe temperatures for filling and emptying low density aerogels with helium.Comment: 8 pages, 5 figures, submitted to PR

Diagnosis and Management of Field Pollution in the Case of an Organochlorine Pesticide, the Chlordecone

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Fluctuating Bond Aggregation: a Model for Chemical Gel Formation

The Diffusion-Limited Cluster-Cluster Aggregation (DLCA) model is modified by including cluster deformations using the {\it bond fluctuation} algorithm. From 3$d$ computer simulations, it is shown that, below a given threshold value $c_g$ of the volumic fraction $c$, the realization of all intra-aggregate bonding possibilities prevents the formation of a gelling network. For $c>c_g$, the sol-gel transition occurs at a time $t_g$ which, in contrast to DLCA, doesnot diverge with the box size. Several results are reported including small angle scattering curves and possible applications are discussed.Comment: RevTex, 9 pages + 3 postscript figures appended using "uufiles". To appear in Phys. Rev. Let

Elasticity near the vulcanization transition

Signatures of the vulcanization transition--amorphous solidification induced by the random crosslinking of macromolecules--include the random localization of a fraction of the particles and the emergence of a nonzero static shear modulus. A semi-microscopic statistical-mechanical theory is presented of the latter signature that accounts for both thermal fluctuations and quenched disorder. It is found (i) that the shear modulus grows continuously from zero at the transition, and does so with the classical exponent, i.e., with the third power of the excess cross-link density and, quite surprisingly, (ii) that near the transition the external stresses do not spoil the spherical symmetry of the localization clouds of the particles.Comment: REVTEX, 5 pages. Minor change

USE OF BIOCHAR TO PROTECT THE VEGETABLES AGAINST THE CHLORDECONE CONTAMINATION: THE STRATEGY OF THE SEQUESTRATION

International audienceIn the case of certain pesticides, the known decontamination methods are not always technically feasible (phyto or bio remediation) or financially and ecologically feasible (excavation, chemical decontamination). We offer an alternative to complete depollution: the sequestration of the pesticide in the soil with the aim of limiting the transfer of the contaminant to the ecosystems. In previous work we have shown that the incorporation of organic matter (compost) in the soil increases the sequestration in the soil of chlordecone. Thus, although contaminated, the soil only releases the pollutant more weakly in the vegetables grown and the water that passes through it. However, organic materials are not permanent and the process must be renewed at intervals of the order of 1 to 2 years. We propose to use Biochar, organic matter stabilized in solid form, which could fulfill the same role of sequestration with a longer duration of effectiveness. This agro-ecological management method (compost, biochars) makes it possible to offer a method that is easy to implement because it is mastered by farmers

SCALING LAW VARIATION OF THE MECHANICAL PROPERTIES OF SILICA AEROGELS

Le comportement mécanique des aérogels de silice est étudié par la méthode de flexion 3 points. Le module d'Young (E), la résistance à la rupture (S) et la ténacité (KIC) sont étudiés en fonction des paramètres de préparation. Il est démontré que l'évolution des caractéristiques mécaniques suit une loi d'échelle en fonction de la densité. Les résultats expérimentaux sont examinés sur la base de la théorie de la percolation.The mechanical behaviour of silica aerogels is studied by the three points flexural technique. The Young's modulus (E), the fracture strength (S) and the toughness KIC are investigated as a function of the different preparation parameters. It is demonstrated that the evolution of mechanical characteristics as a function of the apparent density follows scaling laws. The evaluated exponents are 3.7, 2.6 and 1.6 for E, S and KIC respectively. The experimental data are discussed in terms of the percolation theory

Glasses: Sol–Gel Methods

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EVOLUTION THERMIQUE DES GELS D'ALUMINO SILICATE ALCALIN

Les gels de composition K2O-Al2O3-6SiO2, Na2O-Al2O3-6SiO2, Li2O-Al2O3-6SiO2 ont été préparés suivant des techniques différentes : par destabilisation d'une solution colloïdale de silice et par hydrolyse et polycondensation du Si(OCH3)4. Al2O3, Na2O, K2O, Li2O ont été rajoutés sous forme de nitrates. La cristallisation de ces gels a été étudiée en fonction de la température par diffraction des Rayons X. Si tous les gels présentent à faible température des bandes IR et des pics RX caractéristiques des nitrates, ces derniers se décomposent à plus haute température. La structure devient alors amorphe. Pour des températures plus élevées les gels contenant du potassium présentent la plus faible tendance à la cristallisation. Le retrait du gel est important à basse température il a été suivi par dilatométrie.The gels of composition K2O-Al2O3-6SiO2, Na2O-Al2O3-6SiO2, Li2O- Al2O3-6SiO2 were prepared following two different techniques, destabilization of a colloïdal solution of silica and hydrolysis and polycondensation of Si(OCH3)4. Al2O3, Na2O, K2O, Li2O were added in the form of nitrates in aqueous solution. The crystallization of these gels was studied as a function of temperature by X-Ray diffraction. At low temperature all these gels show characteristic X Ray peaks and also I.R. bands due to the presence of different nitrates. These nitrates however decompose at higher temperatures ; consequently, the structure becomes amorphous. At more elevated temperature the gel containing KNO3 shows a weaker tendency to crystallize. The contraction of the gel was found to be important at low temperatures as observed by dilatometry measurements