DIFFERENT KINDS OF STRUCTURE IN AEROGELS - RELATIONSHIPS WITH THE MECHANICAL-PROPERTIES

The density and the structure (fractal and non-fractal) of aerogels ar e modified either by the adjustment of the gelifying concentration, by a precise control of the viscous how sintering process or by an isost atic pressure deformation. These aerogels have porosities ranging from 98% to 0%. The mechanical properties of the different aerogels (elast ic 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 relati ve density the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. These structural differe nces have been observed by SAXS experiments. The effects of the sinter ing process compared to that of the plastic transformation on the mech anical 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 the ir internal structure is not affected. In addition, relationships betw een structural and porous features and the mechanical properties are d iscussed in terms of percolation theory and the fractal approach. We s how that the exponent of the power law does not depend on the fractal feature and percolation is only an approximation which cannot describe results. (C) 1998 Elsevier Science B.V. All rights reserved.