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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)