ceramics structure

7/28/2019 Ceramics Structure

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Chapter 12- 1

CHAPTER 12: STRUCTURE AND PROPERTIES

OF CERAMICS

How do ceramics differ from metals ?

Keramikos ~ burnt stuff Heat treatment is necessary

Usually a compound between a metal and a non-metal Bonding displays a mixture of ionic and covalent character

Generally hard and brittle, have high melting temperature Why ?

Generally thermally and electrically insulating

Can be opaque, semi-transparent or transparent

Traditional ceramics ~ based on clay (china, porcelain, bricks,tiles) and glasses

Hi-tech ceramics => electronic, communication, computerhardware, aerospace industries


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Chapter 12- 2

Bonding:--Mostly ionic, some covalent.

--% ionic character increases with difference inelectronegativity. What is electronegativity ?

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by

Cornell University.

Large vs small ionic bond character:

CERAMIC BONDING


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Chapter 12-

Crystal Structure of Ionicly Bonded Ceramics

Crystal structure is defined by 2 criterions

1. Magnitude of the electrical charge on each ion. Charge balancedictates chemical formula (Ca2+ and F- form CaF2).

2. Relative sizes of the cations and anions. Cations wants maximumpossible number of anion nearest neighbors and vice-versa.

Stable ceramic crystal structures require anions surrounding acation to be all in contact with that cation.

For a specific coordination number there is a critical orminimum cation/anion radius ratio rC/rA for which this contactcan be maintained. Pure geometrical consideration


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Chapter 12- 3

1. Charge Neutrality:

--Net charge in thecrystal structure

should be zero.

--General form:

2. Maximize the # of nearest oppositely charged neighbors

--stable structures:

Adapted from Fig. 12.1, Callister 6e.

IONIC BONDING & CRYSTAL STRUCTURE


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Chapter 12- 4

Coordination # increases with

Adapted from Table 12.2,

Callister 6e.

Adapted from Fig. 12.2, Callister6e.

Adapted from Fig. 12.3, Callister

6e.

Adapted from Fig. 12.4,Callister 6e.

COORDINATION # AND IONIC RADII


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Chapter 12- 5

On the basis of ionic radii, what crystal structure

would you predict for FeO?

Cation

Al3+

Fe 2+

Fe 3+

Ca 2+

Anion

O2-

Cl-

F-

Answer:rcation

ranion0.077

0.140

0.550

based on this ratio,--coord # = 6

--structure = NaCl (rocksalt)

Data from Table 12.3,

Callister 6e.

EX1: PREDICTING STRUCTURE OF FeO

Two penetrating FCC units; otherexamples are MgO, MnS, LiF.


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Chapter 12- 6

Consider CaF2 :

rcation

ranion0.100

0.133

0.8

Based on this ratio, coord # = 8 and structure = CsCl.

Result: CsCl structure w/only half the cation sites

occupied.

Only half the cation sites

are occupied since

#Ca2+ ions = 1/2 # F- ions.

Adapted from Fig. 12.5, Callister

6e.

EX2: AmXp STRUCTURES

Empty


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Chapter 12-

EX3: ZnS - ZincBlende Structure

Zn2+ + S2-

What is the CN ?

What should be thestructure ?


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Chapter 12-

Ceramic Density Computations

n: number of formula units in unit cell (all ions that areincluded in the chemical formula of the compound =formula unit)

AC: sum of atomic weights of cations in the formula unit

AA: sum of atomic weights of anions in the formula unit

VC: volume of the unit cell

NA: Avogadros number, 6.023 X 1023 (formula units)/mol


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Chapter 12-

EX4: NaCl density

n = 4 in FCC lattice

AC= ANa= 22.99 g/mol

AA= ACl= 35.45 g/mol

VC= a3=[2 (rNa + rCl)]

3

a


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Chapter 12-

Silicate Ceramics

Composed mainly of silicon and oxygen, the two

most abundant elements in earths crust (rocks, soils,

clays and sand- SiO2 silica)

Basic building block: SiO44- tetrahedron:

Si-O bonding is largely covalent, but overall SiO4block has charge of4

Various silicate structures different ways to

arrange SiO44- blocks


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Chapter 12-

EX: Crystalline form of SiO2

Three polymorphs of SiO2 :Quartz, Crystobalite, Tridymite

Not a very closed pack structure

low density ~ 2.65 g/cm3

3D networks of SiO44- tetrahedra

Each O atom is shared byan adjacent tetrahedron


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Chapter 12-

Window Glass Still SiO4

4- tetrahedra are the basicbuilding block.

Most common window glasses areproduced by adding other oxides (e.g.CaO, Na2O, B2O3, etc) whose cationsare incorporated within SiO4 network.

These cations break the tetrahedralnetwork and glasses melt at lowertemperature than pure amorphousSiO2 .

A lower melting point makes it easy toform glass to make, for instance,bottles.

Some other oxides (TiO2, Al2O3)substitute for silicon and become partof the network


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Chapter 12-

Carbon/Diamond/Fullerenes/ Nanotubes

Read => p399-403 http://www.nas.nasa.gov/Groups/SciTech/nano/


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Chapter 12-

Shottky

Defect:

Frenkel

Defect

Frenkel Defect--a cation is out of place.

Shottky Defect

--a paired set of cation and anion vacancies.

Equilibrium concentration of defectsAdapted from Fig. 13.20, Callister 5e. (Fig. 13.20

is from W.G. Moffatt, G.W. Pearsall, and J. Wulff,The Structure and Properties of Materials , Vol. 1,

Structure, John Wiley and Sons, Inc., p. 78.) See

Fig. 12.21, Callister 6e.

DEFECTS IN CERAMIC STRUCTURES

~exp

-QD/kT

A

c

Point defects in ionic crystals are charged. The Coulombic forces that are generated due to defects are very large

and any charge imbalance has a strong tendency to balance itself, electroneutrality. To maintain charge

neutrality several point defects can be created at the same time:

Anion interstitials are

unlikely, why ?

Charge neutrality of

the crystal is

maintained


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Chapter 12- 8

Impurities must also satisfy charge balance

Ex: NaCl

Substitutional cation impurity

Substitutional anion impurity

initial geometry O2-

impurity

O2-

Cl-

an ion vacancy

Cl-

resulting geometry

IMPURITIES


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Chapter 12-

Stoichiometry

A state for ionic solids where there is an exact ratio of

anions to cations defined by the chemical formulaunit.

NaCl => anion to cation ratio is exactly 1:1

Ca2F => 1:2, otherwise it is called nonstoichiometry

FeO => wstite, Fe2+ or Fe3+ may exist depending on

temperature and O partial pressure. For any Fe3+, there hasto be an extra vacancy so that the charge neutrality is

preserved But then, Fe1-xO for x < 1


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Chapter 12-

Impurities in Ceramics

Impurity atoms can exist as either substitutional or as

interstitial solid solutions in ceramics

Substitutional ions substitute for ions of like type (anion toanion, cation to cation)

Interstitial ions are small compared to host structure

formation of anion interstitials is unlikely (why?)

Solubility is higher if ion radii and charges match closely

Incorporation of ion with different charge state requires

compensation by point defects to preserve charge neutrality


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Chapter 12-

Ceramic Phase Diagrams

Al2O3-Cr2O3 system; often they share acommon element in their formula, in many

cases it is OXYGEN.

Solubility is achieved by Al3+ substituting Cr3+

Binary Isomorphous system


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Chapter 12-

Ceramic Phase Diagrams

Al2

O3

-SiO2

system

Composition (wt% alumina)

T(C)

1400

1600

1800

20 00

2200

20 40 60 80 1000

alumina

+mullite

mullite

+ L

mulliteLiquid(L)

mullite

+ crystobalite

crystobalite

+ L

alumina + L

3Al2O3-2SiO 2


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Chapter 12- 9

Room T behavior is usually elastic, with brittle failure.

3-Point Bend Testing often used.--tensile tests are difficult for brittle materials.

Determine elastic modulus according to:

E F

L3

4bd 3

F

L3

12 R4

rect.

cross

section

circ.

cross

section

Adapted from Fig. 12.29,

Callister 6e.

MEASURING ELASTIC MODULUS


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Chapter 12-

Mechanical Properties of Ceramics

Ceramics are very brittle. (Fracture Toughness)

For brittle materials fracture stress concentrators are veryimportant. (Chapter 8: measured fracture strengths are significantlysmaller than theoretical predictions for perfect materials due to thestress risers)

Fracture strength of ceramic may be greatly enhanced by creatingcompressive stresses in the surface region (similar to shot peening,

case hardening in metals, chapter 8) Compressive strength is typically ten times the tensilestrength. This makes ceramics good structural materialsunder compression (e.g., cement, bricks in buildingapartments, stone blocks in the pyramids).

Generally, tensile test is not used Hard to machine, grippers may break the piece, fail after 0.1%strain.

Size is important due impact of # of cracks on strength, why ?


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Chapter 12- 10

3-point bend test to measure room T strength.

FL/2 L/2

cross section

R

b

d

rect. circ.

location of max tension

Flexural strength:

rect.

fs

m

fail

1.5Fmax L

bd 2

Fmax L

R3

Typ. values:Material fs (MPa) E(GPa)

Si nitrideSi carbide

Al oxide

glass (soda)

700-1000550-860

275-550

69

300430

390

69

Adapted from Fig. 12.29,

Callister 6e.

Data from Table 12.5, Callister 6e.

MEASURING STRENGTH


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Chapter 12- 11

Elevated Temperature Tensile Test (T > 0.4 Tmelt).

Generally,

ssceramics

ssmetals

sspolymers. . .

MEASURING ELEVATED T RESPONSE


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Chapter 12- 12

Ceramic materials have mostly covalent & someionic bonding.

Structures are based on:--charge neutrality

--maximizing # of nearest oppositely charged neighbors.

Structures may be predicted based on:--ratio of the cation and anion radii.

Defects

--must preserve charge neutrality

--have a concentration that varies exponentially w/T.

Room T mechanical response is elastic, but fracturebrittle, with negligible ductility.

Elevated T creep properties are generally superior to

those of metals (and polymers).

SUMMARY


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Chapter 12-

Reading: Chapter 12

Core Problems:

Self-help Problems:

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