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Advanced Ceramic
3- ADVANCED FORMING
3.1 SOL – GEL PROCESSING:
The term sol–gel is used broadly to describe the preparation of ceramic
materials by a process that involves the preparation of a sol, the gelation of the
sol, and the removal of the liquid. A sol is a suspension of colloidal particles in a
liquid or a solution of polymer molecules. The term gel refers to the semirigid
mass formed when the colloidal particles are linked to form a network or when
the polymer molecules are cross-linked or interlinked. Two different sol–gel
processing routes are commonly distinguished: the particulate (or colloidal) gel
route in which the sol consists of dense colloidal particles (1 to 1000 nm) and
the polymeric gel route in which the sol consists of polymer chains but has no
dense particles >1 nm. In many cases, particularly when the particle size
approaches the lower limit of the colloidal size range, the distinction between a
particulate and a polymeric system may not be very clear.
Figure 3.1 shows a schematic illustration of the routes that can be followed in
sol–gel processing. The starting compounds (precursors) for the preparation of
the sol consist of inorganic salts or metal-organic compounds, but we shall focus
mainly on the metal alkoxides, a class of metal-organic precursors that are most
widely used in sol–gel research. The chemical reactions that occur during the
conversion of the precursor solution to the gel have a significant influence on
the structure and chemical homogeneity of the gel. A basic problem therefore
understands how the rates of the chemical reactions are controlled by the
processing variables such as chemical composition of the precursor,
concentration of reactants, pH of the solution, and temperature. The problem
becomes more complex when a solution of two or more alkoxides is used in the
fabrication of multicomponent gels (i.e., gels containing more than one metal
cation). There will be a loss of chemical homogeneity if steps are not taken to
control the reaction.
After preparation, the gel contains a large amount of liquid existing in fine
interconnected channels, and it must be dried prior to conversion to a useful
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الرابعة‬
Advanced Ceramic
material. Drying by evaporation under normal conditions gives rise to capillary
pressure that causes shrinkage of the gel network. The resulting dried gel is
called a zerogel. The capillary pressure can be quite large in polymeric gels
because the pores are normally much finer than those in colloidal gels;
consequently, severe problems can be encountered with warping and cracking of
the gel. Two general approaches have been used to circumvent these problems.
The use of chemicals added to the precursor solution prior to gelation, referred
to as drying control chemical additives (DCCAs), permit relatively rapid drying,
but the mechanism by which they operate is not very clear. The removal of the
liquid under supercritical conditions eliminates the liquid–vapor interface and
thereby pre-vents the development of capillary stresses; thus, the gel undergoes
relatively little shrinkage. The dried gel, called an aerogel, is therefore fragile
and may shrink considerably during firing. Most gels have an amorphous
structure even after drying and contain fine pores, which in many cases can be
fairly uniform in size. If a dense ceramic is required as the end product, these
characteristics are very favorable for good densification during sintering. When
compared to the production of similar ceramics by traditional processing routes
involving the compaction and sintering of crystalline particles, a reduction in the
sintering temperature, particularly in the case of polymeric gels, forms a
characteristic advantage of the sol–gel route.
In some cases, crystallization of the gel prior to full densification may limit the
sintering rate for compositions that are crystallizable.
Applications sol–gel processing can provide substantial benefits, such as the
various special shapes that can be obtained directly from the gel state (e.g.,
monoliths, films, fibers, and particles), control of the chemical composition and
microstructure, and low processing temperatures. However the disadvantages
are also real. Many metal alkoxides are fairly expensive, and most are very
sensitive to moisture so that they must be handled in a dry environment (e.g., an
inert atmosphere glove box). The large shrinkage of the gel during drying and
sintering makes dimensional control of large articles difficult. It is often difficult
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to dry monolithic gels thicker than a few millimeters or films thicker than ~1 μm
without cracking. The sol–gel process is therefore seldom used for the
production of thick articles. Instead, it has seen considerable use for the
production of small or thin articles such as films, fibers, and powders, and its
use in this area is expected to grow substantially in the future.
FIGURE 3.1 Schematic illustrations of the routes that could be followed in sol–gel processing.
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