Mechanical Properties of Silica Alcogels and Aerogels
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MECHANICAL PROPERTIES OF SILICA ALCOGELS AND AEROGELS. T. WOIGNIER, J. PHALIPPOU, H. HDACH, L.S.M.V., Universit6 de Montpellier II, 34095 Montpellier Cedex 05, France; and G.W. SCHERER, E.I. DuPont de Nemours & Co, Central R &D Department, P.O. Box 80356, Wilmington, DE 19880-0356. ABSTRACT The mechanical properties of silica gels (alcogels and aerogels) are measured by three point bending. The elastic moduli (E,G) and the fracture strength (a) are investigated as functions of the concentration of silicon precursor, the catalysing conditions, and the aging time. Elastic moduli and strength follow scaling laws as a function of the bulk density of the solid. The scaling exponents are discussed interms of percolation theory. The evolution of the mechanical properties is also followed during the steps of the gel to glass transformation: - hypercritical drying - oxidation
of the organic groups -densification heat treatment At each step of the process the increases of the elastic moduli and a are related to the gel structure. During the hypercritical drying flexible "dead ends" in the network can condense and form new links. The connectivity is also enhanced by the removal of the organic groups creating siloxane bonds. Finally densification leads to a material having mechanical properties identical to those of conventional vitreous silica. I - INTRODUCTION
One of the most important problem studied during this last decade inthe solgel-glass process is the synthesis of bulk monolithic dry gels. Cracks can appear during syneresis, which expels liquid from pores and creates stresses inthe network. The tendency of the gel to crack during drying is attributed to the capillary forces generated in the fragile structure of the gel. Knowledge of the mechanical properties of the alcogel is thus necessary to predict the cracking of the material. Silica aerogel, which is obtained by hypercritical drying (HCD) of the alcogel, is a good candidate as precursor of pure silica glass. This solid material is extremely porous and brittle but only a few studies mention measurements of the strength of a such material. Studies have been performed either on alcogels [1-4] or on aerogels [5-8]. The shear modulus G and the modulus of rupture a of the alcogel increase during aging [2-4] ; in aerogels, Young's modulus E [5-8] and a [8] show a power-law dependence on the bulk density. Inthis work we want to focus our experiments on: - the modification of the mechanical properties G, E and a induced by HCD. - the influence of the experimental conditions, such as concentration of tetramethoxysilane, pH of the hydrolyzing solution, and aging at near ambient temperature on E, G and a for both alcogels and resulting aerogels. All of these parameters induce structural changes, -the evolution of the mechanical properties during the heat treatments leading to the aerogel-glass transformation. II- EXPERIMENTAL PROCEDURE The alcogels are prepared by hydrolysis and polycondensation reactions of tetramethoxysilane (TMOS). The TMOS is dissolved in various amoun
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