Page 152 - Materials Chemistry, Second Edition

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2.4. The Amorphous State

ð48Þ CaCO 3 ! CaO þ CO 2
ð49Þ 3CaO + SiO 2 ! Ca 3 ðSiO 5 Þ }C3S}
ð50Þ 2CaO + SiO 2 ! Ca 2 ðSiO 4 Þ }C2S}

ð51Þ 3CaO + Al 2 O 3 ! Ca 3 Al 2 O 6 }C3A}
ð52Þ 4CaO + Al 2 O 3 þ Fe 2 O 3 ! Ca 4 Al 2 Fe 2 O 10 }C4AF}

2.4.4. Ceramics
Ceramics refer to a broad category of inorganic materials that possess a high
hardness (close to diamond on the Moh’s scale) and brittleness, pronounced resis-
tance to heat and corrosion, and are electrically/thermally insulating. Though amor-
phous glasses may often be classified within the ceramic umbrella, it is generally
accepted to include only inorganic materials with at least some degree of crystallin-
ity. Most often, these materials possess both amorphous and polycrystalline regions,
with the latter exhibiting abrupt changes in crystal orientation across individual
grain boundaries in the extended lattice.
There are three categories of ceramics: oxides (e.g., alumina, zirconia), non-
oxides (carbides, borides, nitrides, silicides), and composites of oxides/non-oxides.
The two common approaches to synthesize ceramics include a low-temperature
sol-gel route (vide supra), and a high-temperature multi-step process involving [88] :
(i) Grinding/milling – silicate/aluminosilicate minerals (e.g., various clays, silica)
and bauxite (hydrous aluminum oxide ore) are among the most abundant
materials in the Earth’s crust, constituting the most common raw materials
for ceramics processing. These minerals are pulverized into a fine powder,
often alongside calcination – thermal treatment to remove volatile impurities
(e.g., organics, waters of hydration, CO 2 from carbonates, etc.).
(ii) Mixing and forming – the powder is mixed with water to semi-bind the particles
together, and is cast, pressed, or extruded into the desired shape. Sometimes, an
organic polymer binder such as polyvinyl alcohol, (—CH 2 —CH(OH)—) n ,is
added to enhance the adhesion of the granules; these additives are then burnt

out during the firing (at T > 200 C). Organic lubricants may also be added
during pressing to increase densification.
(iii) Drying – the material is heated to remove the water or organic binder(s) and
lubricant(s) from the formed material, known as a green body. Care must be
used to prevent rapid heating, to prevent cracking and other surface defects.
During heating, shrinkage will occur resulting in a material that is denser (by a
factor of 2–3) than the green precursor – especially for syntheses using
polymeric precursors (e.g., organosilicon polymers for SiC or Si 3 N 4 ceramics).
Gas evolution may also occur that will introduce porosity into the ceramic.