Page 133 - Materials Chemistry, Second Edition

120 2 Solid-State Chemistry

Metal Alkoxide Solution

Hydrolytic
Polymerization

sol

Furnace Spinning
Spin-coating

Ceramic Fibers
Xerogel Film

wet Gel
Annealing
Solvent Supercritical
Evaporation Solvent Extraction

Dense Film

Aerogel
Xerogel
Figure 2.84. Illustration of the products obtained through sol-gel processing.

reactions. Most often, this step is followed by high-temperature annealing (T > 800
C), in order to convert the amorphous material into a desired crystalline phase. As

an example for alumoxanes, it has been shown that the gel is initially transformed to
g-Al 2 O 3 , en route to its highest crystalline form, a-Al 2 O 3 . [65] During the sintering
(or firing) process, the pore size is also diminished and organic moieties are
removed. Hence, this results in significantly better mechanical properties, as desired
for dense ceramics and glasses.