chapter 12 avi
TRANSCRIPT
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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.
Adapted from Fig. 12.3, Callister6e.
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+
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.
Adapted from Fig. 12.5, Callister6e.
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
Adapted from Fig. 12.29,Callister 6e.
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
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 tothose of metals (and polymers).
SUMMARY
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Chapter 12-
Reading:
Core Problems:
Self-help Problems:
0
ANNOUNCEMENTS