Chemistry Reference
In-Depth Information
CHAPTER 9
Composites
9.1 SOL-GEL CERAMICS
A relatively new area that involves silicon-containing materials is the syn-
thesis of “ultrastructure” materials (i.e., materials in which structure can
be controlled at the level of 100 Å). An example is the “sol-gel” hydrolysis
of alkoxysilanes (organosilicates) to give silica, SiO
2
.
1-20
The reaction is
complicated,
7,
21-26
involving polymerization and branching, but the over-
all reaction may be written
(9.1)
Si(OR)
+→+
2HO iO
4ROH
4
2
2
where the Si(OR)
4
organometallic species is typically tetraethoxysilane
such as tetraethylorthosilicate (TEOS, with R being C
2
H
5
). In this applica-
tion, the precursor compound is hydrolyzed and then condensed to yield
branched polymers. Eventually a continuous swollen gel is formed. The gel
is dried at moderately low temperatures to remove volatile species, and
then it is fired into a porous ceramic object that can then be densified and
machined into a final ceramic part.
The production of ceramics by this novel route triggered interest in the
ceramics community because of advantages over the conventional powder-
processing approach to ceramics. Advantages include (i) the higher purity
of the starting materials, (ii) the relatively low temperatures required, (iii)
the possibility of controlling the ultrastructure to reduce the microscopic
flaws that lead to failure, (iv) the ease with which ceramic coatings can be
formed, and (v) the ease with which ceramic
alloys
can be prepared (e.g., by
hydrolyzing solutions of both silicates and titanates).
The sol-gel approach has been used to form ceramic-like phases in a
variety of polymers. Poly(dimethylsiloxane) (PDMS) is the most popular.
