Page 127 - Materials Chemistry, Second Edition

114 2 Solid-State Chemistry

materials that exhibits an amorphous structure, which our society is indebted to for
countless applications: silica-based glasses. Further, although the majority of
ceramic materials exhibit a crystalline structure, these materials are typically com-
prised of polycrystals alongside embedded amorphous structures. In fact, ceramics
may also have amorphous structures when synthesized at low temperatures, with the
conversion to crystalline phases as their temperature is increased, a process referred
to as sintering, firing,or annealing. This results in the familiar properties of
ceramics such as significant hardness and high melting point, desirable for
structural applications or those occurring within extreme environments such
as high temperatures and/or pressures. In this section, we will describe the
structure and properties of some important classes of amorphous glasses, as
well as partially-amorphous and/or polycrystalline ceramics and cementitious
materials.

2.4.1. Sol-Gel Processing

The sol-gel (solution–gelation) process is a versatile solution-based process for
making ceramic and glassy materials. In general, the sol-gel process involves the
formation of a sol (colloidal suspension of ca. 200 nm solid particles) and
subsequent crosslinking to form a viscous gel. Though this technique has been in
practice since the 1930s, until only recently have the complex mechanisms involved
[62]
in sol-gel been investigated. The most common starting materials, or precursors,
used in the preparation of the sol are water-sensitive metal alkoxide complexes,
M(OR) x , where R ¼ alkyl group (e.g.,CH 3 ,C 2 H 5 ,CF 3 , etc.). Although original
formulations used sodium silicates, the use of alkoxide precursors avoids undesir-
able salt byproducts that may only be removed through long, repetitive washing
procedures. In addition, the nature of the metal and associated R groups may be
altered to affect the rate and properties of the ultimate oxide material.
Sol-gel syntheses are typically carried out in the presence of polar solvents
such as alcohol or water media, which facilitate the two primary reactions of
hydrolysis and condensation (Eqs. 42 and 43, respectively). During the sol-gel
process, the molecular weight of the oxide product continuously increases,
eventually forming a highly viscous three-dimensional network (step-growth
polymerization – Chapter 5).

ð42Þ M OR + H 2 O ! M OH + ROH

ð43Þ M OR + M OH !½M O M] n + ROH
The most widely used metal alkoxides are Si(OR) 4 compounds, such as tetramethox-
ysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides of Al, Ti,
and B are also commonly used in the sol-gel process, often mixed with TEOS. For
instance, aluminum silicates may be generated through hydrolysis/condensation of
siloxides (Eq. 44), which proceed through an intermediate Al–O–Al network known