chapter 12 avi

8/3/2019 Chapter 12 Avi

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Chapter 12-

ISSUES TO ADDRESS...

Structures of ceramic materials:How do they differ from that of metals?

1

Point defects:How are they different from those in metals?

Impurities:How are they accommodated in the lattice and howdo they affect properties?

Mechanical Properties:What special provisions/tests are made for ceramicmaterials?

CHAPTER 12: STRUCTURE AND

PROPERTIES OF CERAMICS


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Chapter 12- 2

Bonding:

--Mostly ionic, some covalent.--% ionic character increases with difference in

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 byCornell University.

Large vs small ionic bond character:

CERAMIC BONDING


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Chapter 12- 3

Charge Neutrality:

--Net charge in thestructure shouldbe zero.

--General form:

Stable structures:--maximize the # of nearest oppositely charged neighbors.

Adapted from Fig. 12.1,Callister 6e.

IONIC BONDING & STRUCTURE


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Chapter 12- 4

Coordination # increases with

Issue: How many anions can youarrange around a cation?

Adapted from Table 12.2,

Callister 6e.

Adapted from Fig. 12.2, Callister6e.



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+

Fe2+

Fe 3+

Ca 2+

Anion

O2-

Cl-

F-

Answer:

rcation

ranion0.077

0.140

0.550

based on this ratio,

--coord # = 6--structure = NaCl

Data from Table 12.3,Callister 6e.

EX: PREDICTING STRUCTURE OF FeO


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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 sitesoccupied.

Only half the cation sitesare occupied since#Ca2+ ions = 1/2 # F- ions.


AmXp STRUCTURES


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Chapter 12- 7

Frenkel Defect

--a cation is out of place.

Shottky Defect--a paired set of cation and anion vacancies.

Equilibrium concentration of defects ~ eQD/ kT

Adapted from Fig. 13.20, Callister5e. (Fig. 13.20 is from W.G.Moffatt, G.W. Pearsall, and J.Wulff, The Structure andProperties of Materials, Vol. 1,Structure, John Wiley and Sons,Inc., p. 78.) See Fig. 12.21,Callister 6e.

DEFECTS IN CERAMIC STRUCTURES


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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- 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


MEASURING ELASTIC MODULUS


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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 carbideAl oxideglass (soda)

700-1000550-860275-550

69

30043039069


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 tothose of metals (and polymers).

SUMMARY


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Chapter 12-

Reading:

Core Problems:

Self-help Problems:

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