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Engineering MaterialsMECH 220

(Ch. 12)


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Topics

Atomic bonding and crystalline structure of ceramics

Common ceramic materials

Point defects in ceramics

Mechanical properties

Topics Covered 12.1-12.5, 12.7, 12.8, 12.9, 12.11


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Ceramicsare compounds of metallic and

non metallicelementswith totally ionic or

predominantly ionic bonds. Ceramicmaterials may have a crystalline or partly

crystalline structure (charge, size),or may

be amorphous (e.g., a glass).

The word "ceramic" comes from the Greek

word (keramikos), burnt stuff".

China clay, bricks, porcelain, tiles, glass etc...


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P4 Al2Si2O5(OH)4.
http://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Aluminium


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Structural- bricks, pipes, floor, roof tiles.

Refractories - such as gas fire radiants, steel and glass

making crucibles.

Whitewares - including tableware, cookware, wall tiles,pottery products and sanitary ware.

Engineering - Such items include tiles used in the Space

Shuttle program, gas burner nozzles, ballistic protection,

nuclear fuel uranium oxide pellets, biomedical implants,coatings of jet engine turbine blades, ceramic disk brake,

missile nose cones, bearing (mechanical).

Categories of Ceramic Materials


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Si3N4 bearing and partsClassification of engineering ceramics


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Classification of engineering ceramics


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The Porsche Carrera GT's carbon-

ceramic (silicon carbide) composite disc

brake

http://en.wikipedia.org/wiki/File:PCCB_Brake_Carrera_GT.jpg


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AerospaceEngines components; Airframes; Missile nose-cones; Space Shuttle tiles;

ballistic shields; Rocket nozzles.

AutomotiveHeat shield; Exhaust heat management devices

Biomedical

hip prosthesis ceramic head; Artificial bone; Dentistry applications, teeth;Biodegradable splints; Implant materials.

ElectronicsCapacitors; Integrated circuit packages; Transducers; Insulators

OpticalOptical fibers; Laser amplifiers; Lenses; Infrared heat-seeking devices
http://en.wikipedia.org/wiki/File:Hip_prosthesis.jpghttp://en.wikipedia.org/w/index.php?title=Ceramic_engineering&action=edit&section=18http://en.wikipedia.org/w/index.php?title=Ceramic_engineering&action=edit&section=19http://en.wikipedia.org/w/index.php?title=Ceramic_engineering&action=edit&section=17http://en.wikipedia.org/wiki/File:Hip_prosthesis.jpg


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Atomic bonding in ceramics

Ceramicsconsist of two or more chemical elements

Depending on the difference in electronegativity, atomicbonding can be predominantly ionicor covalent

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SiC: Covalent

CaF2: Ionic


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Crystal structure of Ionicceramics

Most common ceramics are crystalline

The crystalline structure is such that: The structure is electrically neutral (equal number of + andcharges)

Each cation (+ve) has as many closest anion (-ve) neighbors as

possible

The number of closest neighbors of an atom is the

coordination number

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Coordination number = 2 Coordination number = 3 Coordination number = 4

Stable structures: cation-

anion contact

Linear

Planar triangle

Tetrahedral


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Crystal structureIonic ceramics

A cation is smaller than an anion:

The bigger the cation the higher the number of possiblenearest neighbors the coordination number depends on

Effect of ion charge on the radius (e.g. Fe2+, Fe3+)

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1c

a

r

r

rc / ra

Cation radius

Anion radius


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Example: for coordination number 3?rc/ r

a

Answer = 0.155

(1) Geometrical

considerati

ons

(2) hard sphere

model

Computation of Minimum Cation-Anion Radius Ratio


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Example: for coordination number 6?rc / ra

Answer = 0.414


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Determine minimum rcation/ranionfor an octahedral site(C.N. = 6)

a=2ranion

2ranion 2rcation= 2 2ranion

ranion rcation= 2ranion rcation= ( 21)ranion

arr 222 cationanion =

414.012

anion

cation==

r

r

Solution


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Example problem 12.2

Based on table 12.3, what is the crystal structure for FeO?

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Ionic radius (nm)

0.053

0.077

0.0690.100

0.140

0.181

0.133

Cation

Anion

Al3+

Fe2+

Fe3+Ca2+

O2-

Cl-

F-

0.0770.55

0.140

c

a

r

r= =

Based on Table 12.2 C.N.= 6


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Crystal Structures where cationand anion have the same charge


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Rock salt (or NaCl) crystalline structure

Each cation has six anion closest neighbors => CN = 6

The anions have the same

coordination number as cations

rc/rA=0.414-0.732

Cations and anions are

arranged in FCCcells

(interpenetratingFCC cells)

Examples: NaCl, MgO, FeO

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Cesium chloride structure (CsCl)

Each cation has 8 anion nearest neighbors (cubic site,

CN=8 for both ions)

Anions occupy the corners of a cube, a cation occupies the

center

What type of structure is this one??BCC or FCC??

The anions and cations occupy simple

cubic lattice sites

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Zinc blende (sphalerite, ZnS) crystalline

structure Each cation has 4 anion nearest neighbors (tetrahedralsite, CN = 4)

Anions occupy the corners and

faces of a cubic cell (???)

Cations occupy tetrahedral positions

Examples: ZnS, SiC

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Cations

Anions

Source: wikipedia


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Crystal Structures where cationand anion have different charge


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AmXpType Crystal Structure

If the charges on the cation and

anion are not same a compoundexists with a formula AmXpwhere

m and/or p 1

Example: Calcium fluorite CaF2,

the rc/raratio is about 0.8 and

coordination number 8. Ca ions

are at the center of the cube and F

ions at the corners.

Fluorite structure: CeO2, ZrO2, UO2


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ABX3-type structure, twocations

A and B for 3 anions X Example: BaTiO3(Barium

Titanate)

The smaller cation has room to

move within the cell interesting

electric and magnetic properties

At high temperature (>120 C),

BaTiO3belongs to the cubic

system

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AmBnXpType Crystal Structure

Perovskite structure

What is the number of Ba2+, Ti4+, O2-ions in the unit cell?????


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Theoretical Density computation from Unit cell dat

The density is given by

n: number of formula units (ex. NaCl, MgO, ) within unit cell

M: atomic mass of formula unit

Ac: atomic mass of a cation within unit cell

Aa: atomic mass of an anion within unit cell

Vc: volume of unit cell

NA: Avogadros constant

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//=

c a AA

c c

n A A N n M N

V V

=


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Example problem 12.3 (Self Study)

Determine the density of NaCl

ANa= 22.99 amu, ACl= 35.45 amu, NA= 6.022 x 1023 atoms/mol

Rock salt structure

both Na, Cl occupy FCC lattice sites

One FCC cell 4 atoms

Edge length:

From table 12.3:

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23 33

4 22.99 35.45 / 6.022 10= 2.14 g/cm

a

=

2 2Cl Na

a r r =

0.181 , 0.102Cl Na

r nm r nm = =


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Common ceramics - Silicates

Silicates are ceramics consisting of silicon and oxygen.

These are the most common chemical elements on earth The simplest form of silicates is SiO4

4-

SiO2is silica. It has a number of polymorphic

formsthat include quartz, crystobalite, & tridymite

Glass is an amorphous form of SiO2

The Si-O bonds are strong & covalent (directional) high

melting temperature~1700 C

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Bonding of adjacent SiO4

4-accomplished by the sharing

of common corners, edges, or faces

Silicates

Mg2SiO4 Ca2MgSi2O7

Adapted from Fig. 12.12,

Callister & Rethwisch 8e.

Presence of cations such as Ca2+

, Mg2+

, & Al3+

1. maintain charge neutrality, and

2. ionically bond SiO44-

to one another


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The structure of glass

Glass is amorphous (non-crystalline)

Basic unit is SiO44-

Common glass can be Pure fused silica

Fused silica with impurities, such as Na+, Al3+

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Quartz is crystalline

SiO2:


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Layered Silicates

Layered silicates (e.g., clays, mica, talc)

SiO4 tetrahedra connectedtogether to form 2-D plane

A net negative charge is associated witheach (Si2O5)

2-unit

Negative charge balanced byadjacent plane rich in positively chargedcations




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Kaolinite clay alternates (Si2O5)2-layer with Al2(OH)4

2+

layer

Layered Silicates (cont.)

Note: Adjacent sheets of this type are loosely bound to one another by van derWaals forces.

Adapted from Fig. 12.14, Callister &

Rethwisch 8e.

OH-, O2-

Th f b


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The case of carbon

Carbon is not a ceramic, but graphite is somet imes classified

as a ceramic

Graphite has a number of polymorphs, including

Diamond: has the crystalline structure of Zinc blende

Fullerenes (C60): has a sphere-like structure containing 60 carbon atoms

Carbon nanotubes (CNT): tubular form, impressive mechanical properties

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GraphiteDiamond

C60

CNT

Spherical cluster

Di d


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Diamond

Carbon organized in tetrahedrons

Strong bonds hardest known material Very high thermal conductivity (> Cu)

Can exist as large single crystals

(gemstones!)

Small crystals are used forgrinding/cutting other materials

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weak van der Waals forces between layersplanes slide easily over one another -- good lubricant

Graphite

P i t d f t i i


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Point defects in ceramics

Point defects include vacancies, interstitial defects, and

substitutional defects Vacancies: missing cations or anions

Interstitial defect: extra cation inside interstice

Anions are too large for interstitial sites

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Cation interstiti

Cation vacancy

Anion vacancy

Electroneutralityshould be applied

[+]=[-]

S i l t f i t d f t


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Special types of point defects

Frenkel defect: cation vacancy AND cation interstitial

Schottky defect: cation vacancy AND anion vacancy

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Shottky

Defect:

Frenkel

Defect

Neutrality of electriccharge is preserved

Schottky

defect

Frenkel

defect

Stoichiometry is preserved:ratio of cations/anions is the samewith the formula ratio

Non stoichiometry: Fe1-XO instead of FeO (Fe2+, Fe3+states)

C ti th b f d f t


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Computing the number of defects

Number of Frenkel defects

Number of Schottky defects

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Nfr

=Nexp Qfr2kT

Ns =Nexp Qs

2kT

Activation energy

Number of lattice sites(potential defects)

Boltzmanns constant

Temperature (K)

C i Ph Di


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Ceramic Phase Diagrams

MgO-Al2

O3

diagram (as an example):



Spinel phase for a

range of

compositions

Low solubility of

Al2O3 to MgO

below 1400oC

MgO insoluble to

Al2O3

2 eutectic points


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F t f i (F t h f i )


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Fracture of ceramics (Fractography of ceramics)Crack propagation study and microscopic features of the

fracture surface

Crack propagates until a critical velocity is reached (e.g. for glass is

half of the speed of sound) and this is repeated until a family of cracks

is formed.

Fl l ( b d) t th


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Flexural (or bend) strength

Stress-strain response of ceramics is difficult to determine using

a tensile experiment because of brittleness (fail after 0.1% ofstrain)Bending (flexure) test is applied instead

Flexural strength is the stress at fracture using a flexure test Specimen (rectangular of circular geometry), 3 or 4-point technique

For a beam of rectangular cross-section:

For a beam of circular cross-section:

Ff: Load at fracture, L: length of beam, R: radius, b: width of beam, d: height

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232

f

fsF Lbd

=

3

f

fs

F L

R

=

Compression state

Tension state


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Influence of porosity on mechanical properties


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Influence of porosity on mechanical properties

Elastic stiffness (Youngs modulus) decreases with

increasing porosity (pores due to powder ceramic fabrication,forming, etc.)

Flexural strength decreases exponentially with porosity (pores

reduce the actual cross sectional area where load is applied

+they act as stress concentrators)

20 1 0.9 0.9E E P P=

Stiffness of non-porous material Volume fraction of pores

0 exp( )fs nP =

Flexural strength of non-porous material Material parameter

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