Ceramics

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Ceramics
Ceramics are inorganic, nonmetallic, solids,
crystalline, amorphous (e.g. glass). Hard,
brittle, stable to high temperatures, less dense
than metals (up to 40). More elastic than
metals (do not readily deform under
stress). Very high melting (up to 2800oC).
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Ceramics
Ceramics can be covalent-network and/or ionic
bonded. Typical examples Aluminates alumina
(Al2O3) Carbides silicon carbide
(SiC) Oxides zirconia (ZrO2) and beryllia
(BeO) Silicates silica (SiO2)
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Ceramics
Processing of Ceramics Ceramics are very brittle
and shatter when struck. Bonding prevents atoms
from sliding over one another. Compare steel and
a clay pot!
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Ceramics
Processing of Ceramics Small defects
(micro-cracks and voids) developed during
processing make ceramics weaker. Micro-crack
areas more susceptible to more stress. Minimise
stress fractures by using very pure uniform
particles (lt 1?m or 10-6m in diameter).
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Ceramics
Processing of Ceramics Sintering Heating of
very pure uniform particles (about 10-6 m in
diameter) under high temp pressure to force
particles to bond. During sintering the
particles coalesce without melting. Alumina
(Al2O3) melts at 2050oC but coalesces at 1650oC
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Ceramics
Processing of Ceramics Sol-gel process
formation of pure uniform particles. Metal
alkoxide is formed (e.g. Ti(OCH2CH3)4). Sol
formed by reacting metal alkoxides with water (to
form Ti(OH)4).
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Ceramics
Processing of Ceramics Ti(s) 4CH3CH2OH(l) ?
Ti(OCH2CH3)4(s) H2(g) Ti(OCH2CH3)4 4H2O(l) ?
Ti(OH)4 4CH3CH2OH(l) Direct addition of water
to Ti(s) leads to complex mixtures of oxides and
hydroxides. Alkoxide intermediate ensures a
uniform suspension of Ti(OH)4. This is the sol
stage.
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Ceramics
Processing of Ceramics Acidity or basicity of
the sol is adjusted to split water from between
two of the Ti-OH bonds. (HO)3Ti-O-H(s) H-O-Ti(O
H)3(s) (HO)3Ti-O-Ti(OH)3(s) H2O(l) This is
another example of a condensation reaction.
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Ceramics
Processing of Ceramics Condensations also occurs
at some of the other OH groups. This produces a
three-dimensional network This is the Gel and
the suspension of extremely small particles has
the consistency of gelatine.
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Ceramics
Processing of Ceramics Gel is heated carefully
at 200oC to 500oC to remove water and the gel is
converted into finely divided oxide
powder. Particle size range of 0.003 to 0.1mm in
diameter.
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Ceramics
Applications of Ceramics Used in cutting tool
industry (alumina reinforced with silicon
carbide). Used in electronic industry
(semiconductor integrated circuits usually made
of alumina).
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Ceramics
Applications of Ceramics Piezoelectric materials
Quartz (crystalline SiO2) Generates an
electrical potential after mechanical stress and
is used in watches and ultrasonic generators.
Heat resistant tiles (Silica fibres reinforced
with aluminum borosilicate fibres) Tiles (0.2
g.cm-3) on the space shuttle has surface
temperature of 1250oC while shuttle surface
temperature gets to 180oC during re-entry.
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Ceramics
Superconductors show no resistance to flow of
electricity. Superconducting behavior only
starts below the superconducting transition
temperature, Tc.
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Ceramics
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Ceramics
Meissner effect permanent magnets levitate over
superconductors. The superconductor excludes all
magnetic field lines from its volume, so the
magnet floats in space.