Civil Engineering Reference
In-Depth Information
silica aerogels has recently been written by Dorcheh and Abbasi (2008) and
the author would like to refer to this work for a more extensive analysis of
the aerogel synthesis.
Gel preparation
Gel preparation happens by means of a sol-gel process (Brinker & Scherer,
1990; Dorchech & Abbasi, 2008), i.e., a process in which solid nanoparticles
dispersed in a liquid agglomerate together to form a continuous three-
dimensional network extending throughout the liquid. The main precursors
for silica aerogels are silicon alkoxides. A simplifi ed reaction based on tet-
ramethoxysilane may be presented as
Si(OCH
3
)
4
+
2 H
2
O
→
SiO
2
+
4 CH
3
OH
The sol becomes a gel when the dispersed solid particles stick together
during collision to form a network of particles spanning the entire liquid.
Nanoparticles containing reactive surface groups stick by bonding or elec-
trostatic forces, whereas others may require an additive. In general, acid
hydrolysis and condensation results in weakly branched chains and micro-
porous structures in silica sols and resulting long gelation times, whereas
uniform particles are easily formed in base catalysis, leading to a broader
distribution of larger pores which is less favorable for thermal insulation
materials.
Aging of the gel
Aging of the gel in its mother solution is required to prevent the gel from
shrinking during drying (Haereid
et al.
, 1996). The silica spine of the gel
contains a signifi cant number of unreacted alkoxide groups, and hydrolysis
and condensation must continue a suffi cient time for strengthening of the
silica network. During this aging period, material transports to the spine
neck region and small particles dissolve into larger ones. Common aging
procedures involve ethanolsiloxane mixtures, adding new monomers to the
solid SiO network, and increasing the degree of cross-linking. After aging,
all water remaining within the pores must be removed before drying: any
water left in the gel leads to an opaque and very dense aerogel.
Drying of the gel
Drying of the gel is the most critical stem in the production process and
happens under special conditions to prevent the gel structure from collaps-
ing due to shrinkage and possible capillary tension in the small pore sizes.
If a liquid is held under pressure always greater than the vapor pressure,
Search WWH ::
Custom Search