Geoscience Reference
In-Depth Information
involving very high temperature/energy and cost, which in turn contribute to global
warming. These materials may be disposed off on the earth or recycled [420] .
A number of substances with particular compositions, crystal structures, and
specified properties have been investigated in detail by various workers all over the
world. However, only a few materials can be considered as useful materials,
because some substances which have desired composition structures and properties
cannot be used as they are extremely difficult to give desired shapes or forms
(this important point has often been overlooked). Particularly, it is difficult to
give desired shapes, forms, and size, to inorganic materials owing to their high brit-
tleness. Organic materials, such as polymers and plastics, or metallic materials can
be generally deformed when local stresses (above their yield stresses) are applied
to them, but inorganic materials, particularly ceramics, are likely to break due to
brittle fracture. Because ceramics have generally been fabricated by a rather special
“ceramic processing” which consists of two steps: (1) Synthesis of powders and (2)
shape forming by firing/sintering of the powders or melting (in the case of glasses),
both the steps usually require high temperatures and consume a lot of energy.
10.9.1 Thermodynamic Principles of Advanced Materials Processing
Processing of advanced materials generally consists of two steps: (1) Synthesis of
substances (ceramic, metallic, organic) which have a particular chemical composi-
tion, structure, and properties and (2) materials fabrication (i.e., shape forming by
firing/sintering, melting, molding, or casting). Organic materials, like polymers and
plastics, or metallic materials can generally be deformed when local stresses (above
their yield stresses) are applied to them [421,422] , but ceramics are likely to break
due to brittle fracture [423] . The two steps in “classical” processing usually require
high temperatures, thus consuming a lot of energy, particularly in the case of cera-
mics [424] . More recent processing routes, using a gaseous phase, like CVD and
MOCVD [425,426] , or vacuum systems such as sputtering and MBE [425,427] ,
require much more energy than standard high-temperature processing. Generally,
all these techniques have resulted in environmental problems because the consumed
energies are emitted as exhaust gas(es) or exhaust heat (entropy) except for the part
involved in the production. Especially, the vacuum systems seem to be worse
because they need continuous pumping to maintain the vacuum and their exhaust
gas(es) cannot be cycled due to their diluted huge volumes.
The total energy consumption among all the mentioned processing routes should
be the lowest in aqueous solution systems, because an excess of energy is necessary to
create melts, vapor, gas, or plasma, than to form aqueous solutions at the same temper-
ature (Figure 1.9) [417] . This idea can be demonstrated using an example of BaTiO 3 ,
which is one of the most important materials for the electronic industry. Driving force
(
Δ
G) for representative synthesis reactions of the BaTiO 3 Eqs (10.2)
(10.6) are
38 kcal/mol, 3685 kcal/mol, 17 kcal/mol, and
14 kcal/mol, respectively, at room
temperature [167,428] . Figure 10.40 shows the energy diagram for the formation of
BaTiO 3 from various precursors.
2
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