Mechanical properties of gel-derived materials

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 elast ic and rupture moduli which are 10(4) times lower. However, if this analogy is pertinent when gels are under a tension stress (bending test) they exhi bit a more complicated response when the structure is submitted to a compre ssive stress. The network is linearly elastic under small strains, then exh ibits yield followed by densification and plastic hardening. As a consequen ce of the plastic shrinkage it is possible to densify and stiffen the gel a t room temperature. These opposite behaviours (elastic and plastic) are sur prisingly related to the same two kinds of gel features: the silanol conten t and the pore volume. Both elastic modulus and plastic shrinkage depend st rongly on the volume fraction of pores and on the condensation reaction bet ween silanols. On the mechanical point of view (rupture modulus and toughne ss), 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 por e size distribution. Different kinds of gels structure (fractal or not frac tal) have been synthesized by a control of the different steps of transform ation such as sintering and plastic compaction. The relationships between s tructural and the elastic properties are discussed in terms of the percolat ion theory and fractal structure.