Civil Engineering Reference
In-Depth Information
size: 0.3-2 nm
10
m
μ
10.1
Structure of the nanoporous SiO
2
network of silica aerogel (Reim
et al.
, 2005).
Gel ageing
The gels are usually aged before drying, in order to increase stiffness and
strength and to mechanically reinforce the tenuous solid skeleton generated
during the sol-gel process. Different ageing techniques can be used. They
are based on modifying the composition of the liquid phase contained in the
pores by adding water and/or monomeric alkoxylanes. In fact, a signifi cant
number of the alcoxides contained in the liquid could be unreacted. The
addition of these substances can enhance the surface reactions and allow
supplementary condensation and re-precipitation of silica on the skeleton.
The mechanisms operating during the ageing phase are the neck growth
from reprecipitation of silica dissolved from particle surface onto necks
between particles, and the dissolution of smaller particles and precipitation
onto larger ones. The consequence is generally an increasing of the average
pore size and of the apparent density of the gel. After ageing, the water still
within the pores must be removed, by washing the gel with ethanol and
heptanes. If water remains in the pores, it will not be removed with super-
critical drying, therefore it will lead to an opaque and very dense gel.
Gel drying
Aerogels are the solid framework of the sol-gel isolated from liquid by
means of the drying phase; it is the most critical step of the process, because
it is governed by the capillary pressure which can cause fractures or collapse
in the structure. Two different methods are usually used: ambient pressure
drying (APD) and supercritical drying (SCD), where the capillary tension
can be avoided by removing the liquid above the critical temperature and
pressure. APD is generally carried out in two steps:
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