ceramics and glass
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CERAMICS& GLASS
REFF:
Materials Science & Engineering; An Introduction
Callister, W. D, Jr, 2007, John Wiley & Sons Fundamental of Ceramics, Barsoum, M. W., 2003,McGraw-Hill
Engineering Materials 2; An Introduction toMicrostructures, Processing and Design, Ashby, M. F
and Jones, D. R. H, 1986, Pergamon Press
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Introduction
CERAMICS: Greek keramikos = burn stuff
solid compounds formed by heat (&/P)
applications followed by coolingat least 2 elements; 1 is a non-metal, the
other may be (a) metal(s) or (an)other non
Oxygen and silicon are abundance in nature:rocks, dust, clay, mud
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Atom arrangement
In ceramics: long-range
order, short-range order or
both combination
Long-range order: repeatperiodicity >>bond length
cryatalline solid;ceramics
Short-range order:No
periodicity amorphous,
glassy or noncrystalline
solid; glass
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1 unit cell: the smallest
region when repeated,
completely describes3D atoms of crystal
Crystalline solid: single
crytals & polycrystalline
solids
Single crystal: solid in
which the periodic &
repeated arrangementof atom is perfect &
extends specimen
entire, no interruption
Polycrystalline solid:
many single crystals
(grains), separatedeach other by disorder
area (grain boundaries)
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General Characteristics of
Ceramics
Hard
Wear-resistant
Brittle
Prone to thermal
shock
Resistant to usual
treatment Oxidation resistant
Chemically stable
Nonmagnetic
Electrically &
thermally insulative
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Ionic Vs Covalent Bonding
Ionic compound form between very active
metallic & nonmetallic elements
To form AX ionic bonding, A loses e easily,
X accepts e without too much energy input
Covalent bonding occurs if energies of
bonding electrons of A & X are comparable
If the electron energy of the atoms is
different transfer energy (ionic bonding)
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Ceramic Structures
Atomic bonding: purely ionic to totally
covalent, many exhibit combination type
Degree of ionic character
electronegativities of atoms
Electronegativity (e greed): the power of
atom to attract electrons to itself
Two elements forming a bond have similar
electronegativity share e; covalent bond
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Mutual attraction
lower potential energy If the electronegativity difference between
them (x)is large (indicating 1 element is
greedier than other), e attracted to the
more electronegative elemention attract
each other
x > 1.7ionic
x < 1.7covalent
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Character of ionic compounds: have high
melting & boiling points (the bonds are strongand omnidirectonal), hard, brittle, poor electrical
and thermal conductors
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Crystal Structure
Ionic bondingcomposed of electricallycharge ions instead of atoms
Metalic ions (cation, + charge) give
valence electron to the nonmetalic Characteristics of ions which affect crystal
structure:
1. magnitude of electrical charged of eachions
2. relative size of + and - ion
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Crystalelectrically neutral; (+) charges
must be balanced by an equal number of
() related to the chemical formula
Each cation prefers as many neighbour
anions, anions also desire a maximum
number of cation. Coordination no. (number of anions
neighbors for a cation)related to rc/ra
Stable structures require that cations andanions touch
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Table: Coordination
numbers and
geometries for
various rc/ra Blue cation
Redanion
Common coordinationnumbers for ceramic:
4, 6 and 8
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The size of an ion depend on:
1. coordination number
Ionic radius increase as the number of opposite
charge neighbor ions increases2. number ofe of atom/ion
Removing e from atom/ion, the remainingvalence electrons become more tightly bound to
the nucleus decrease ionic radius. Ionic sizeincreases when electrons are added to an atomor ion.
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AX-type crystal structures
Ceramic of metal & nonmetal element (AX)
with equal number of A (cation) & X (anion)
Ionic MgO; 2 e of A transferred to X, result
in Mg2+ & O2-
CovalentZnS; sharing elektron
3 structures: rock salt, CeCl and ZnS
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Rock salt (NaCl) structure
The most common AX crystalstructure
Electrostatic attraction betweenNa+ & Cl- hold the crystaltogether
Max. electrostatic interactioneach Na+ has 6 Cl-, no Na+neighbours (vice versa)
Coordination number for both +
& - is 6 (octahedral) 1 unit cell generated from
FCC of anion with 1 cation incubic center & 1 at centered ofeach of 12 cube edge
NaCl, MgO, MnS, LiF and FeO
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Cesium cloride (CsCl) stucture
Coordination number
for both ions is 8
(cubic)
The anions are ateach of the corners of
a cube
Single cation is at the
cube center
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Zinc Blende (ZnS) structure
Coordinate number for
both ions is 4
(tetrahedral)
all corner and face
positions of the cubic cell
are occupied by S atoms
the Zn atoms fill interior
tetrahedral positions
Most often the atomic
bonding is highly covalent
ZnS, ZnTe, and SiC
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SILICATE CERAMIC
Material composed primarily of silicon and oxygen
Each silicon atom bond strongly to 4 oxygen atom
Basic unit in all silicates tetrahedron (oxygen are
situated at the corners, oxygen is at the centre)
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The most simple: silicondioxide/silica
Pure silica no metal ions,every oxygen becomes abridge between 2 silicon atoms
Every corner oxygen atom isshared by adjacent tetrahedra
The materials electricallyneutral, all atoms have stableelectronic structures
Ratio Si to O1:2 (indicatedby chemical formula)
If tetrahedras are arranged in aregular & order crystalline
3 polymorphic: quartz,cristobalite & tridymite
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Tetrahedra link: directly or viametal ion (M)
Silicas combined with metaloxides (e.g. MgO, CaO,
Al2O3) MO/SiO2 2separated
SiO4 monomer linked by MOmolecules
MO/SiO2 < 2 silicadimers;1oxygens sharedbetween 2 tetrahedra(bridging oxygen)
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Decrease amount of MO increase degree ofpolymerisation
Chain form
linked tetrahedra BackboneSi-O-Si-O-Si-;
2 oxygens of each tetrahedron are shared (2bridging oxygens)
The others forms ionic bonds between chains,joined by MO; weaker than backbonefibrous
E.g. asbeston
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Sheet form3 oxygen of each tetrahedron are
shared; e.g. clay
M attached to one of side of sheet (side withspare oxygen on it)
The sheet is polarised a net + charge on one
surface, - charge on the other This interaction strongly with water, attract a
water layer between the sheets clay plastic
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Silica Glasses
Commercial glasses are based on silica Silica also exist as noncrystalline solid/glassfused
silica/vitreous silica
Pure silica forms glass with high softening T (1200 C)
Great strength and stability, low thermal expansion but hard to
work with because high in viscosity
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Commercial glasses silica glasses add with
other metal oxide toreduce viscosity
E.g. CaO, Na2O addpositive ion to the
structure &break up thenetworknetworkmodifiers
Add 1 Na2O molecules
introduces 2 Na+, eachattaches to 1 oxygen oftetrahedronnonbridging
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TiO2, Al2O3 substitute silicon; become partand stabilize the network intermediates
Reduction in cross-linking/ substitute silicon
soften the glass, reduce Tm, viscosity of glass easier to form at low T
Common window glass: 70% SiO2, easily workat 700 C
Pyrex: 80% SiO2, less modifier, better thermalshock resistance, thermal expansions lower,harder work (800 C)
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