Mechanical Properties of Aerogels : Brittle or Plastic Solids? |
| Journal |
Key Engineering Materials (Volume 391) |
| Volume |
Progress in Sol-Gel Production |
| Edited by |
Luis Esquivias |
| Pages |
27-44 |
| DOI |
10.4028/www.scientific.net/KEM.391.27 |
| Online since |
October, 2008 |
| Authors |
Thierry Woignier,
A. Hafidi Alaoui,
Juan Primera,
J. Phalippou
|
| Keywords |
Aerogel, Elastic Property, Mechanical Property, Plastic Behaviour, Toughness, Weibull Statistics |
| Abstract |
Different sets of silica aerogels (classical aerogels, partially dense aerogels, composite aerogels)
have been studied in the objective to understand the mechanical behaviour of these extremely
porous solids. 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 modulus 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 compact and stiffen the gel at
room temperature. These opposite behaviours (brittle and plastic) are surprisingly related to the
same kinds of gel features: pore volume silanol content and the pore size. 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 size plays likely 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. |
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