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Chapter 12: & Properties of

ISSUES TO ADDRESS... • Structures of materials: How do they differ from those of ? • Point defects: How are they different from those in metals? •Impurities: How are they accommodated in the and how do they affect properties? • Mechanical Properties: What special provisions/tests are made for ceramic materials?

Chapter 12 - 1 Ceramic Bonding • Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. • Large vs small ionic bond character:

CaF2: large SiC: small

Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from , The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Chapter 12 - 2 Ceramic Structures

Oxide structures – anions much larger than cations – close packed oxygen in a lattice (usually FCC) – cations in the holes of the oxygen lattice

Chapter 12 - 3 Site Selection Which sites will cations occupy? 1. Size of sites – does the cation fit in the site 2. Stoichiometry – if all of one type of site is full the remainder have to go into other types of sites. 3. Bond Hybridization

Chapter 12 - 4 Ionic Bonding & 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. ------Adapted from Fig. 12.1, + + + Callister 7e. ------unstable stable stable • Charge Neutrality: F- --Net charge in the 2+ structure should CaF : Ca + 2 cation anions be zero. F- --General form: AmXp m, p determined by charge neutrality Chapter 12 - 5 Coordination # and Ionic Radii rcation • Coordination # increases with r anion TD:TETRAHEDRA Issue: How many anions can you arrange around a cation?

rcation Coord ZnS (zincblende) ranion # Adapted from Fig. < 0.155 2 linear 12.4, Callister 7e.

0.155 - 0.225 3 triangular NaCl ( 0.225 - 0.414 4 TD chloride) Adapted from Fig. 12.2, Callister 7e.

0.414 - 0.732 6 OH CsCl (cesium chloride) 0.732 - 1.0 8 cubic Adapted from Fig. Adapted from Table 12.2, 12.3, Callister 7e. Callister 7e. Chapter 12 - 6 Cation Site Size

• Determine minimum rcation/ranion for OH site (C.N. = 6)

2 ranion  2rcation  2a

a  2ranion

2 ranion  2rcation  22ranion

r anion  rcation  2ranion r cation  (21) ranion r cation  0.414 ranion Chapter 12 - 7 Site Selection II

2. Stoichiometry – If all of one type of site is full the remainder have to go into other types of sites.

Ex: FCC has 4 OH and 8 TD sites.

If for a specific ceramic each unit cell has 6 cations

and the cations prefer OH sites

4 in OH

2 in TD

Chapter 12 - 8 Site Selection III

3. Bond Hybridization – significant covalent bonding – the hybrid orbitals can have impact if significant covalent bond character present – For example in SiC

• XSi = 1.8 and XC = 2.5

2 % ionic character  100 {1- exp[-0.25( XSi  XC ) ]}  11.5% • ca. 89% covalent bonding • both Si and C prefer sp3 hybridization

• Therefore in SiC get TD sites

Chapter 12 - 9 Example: Predicting Structure of FeO • On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Ionic radius (nm) •Answer: 3+ Al 0.053 rcation 0.077 2+  Fe 0.077 ranion 0.140 3+ Fe 0.069  0.550 Ca2+ 0.100 based on this ratio, --coord # = 6 Anion --structure = NaCl O2- 0.140 Cl- 0.181 Data from Table 12.3, F- 0.133 Callister 7e. Chapter 12 - 10 Rock Salt Structure

Same concepts can be applied to ionic in general. Example: NaCl (rock salt) structure

rNa = 0.102 nm

rCl = 0.181 nm

rNa/rCl = 0.564

 cations prefer OH sites

Adapted from Fig. 12.2, Callister 7e.

Chapter 12 - 11 MgO and FeO

MgO and FeO also have the NaCl structure

2- O rO = 0.140 nm

2+ Mg rMg = 0.072 nm

rMg/rO = 0.514

 cations prefer OH sites

Adapted from Fig. 12.2, Callister 7e. So each oxygen has 6 neighboring Mg2+

Chapter 12 - 12 AX Crystal Structures

AX–Type Crystal Structures include NaCl, CsCl, and blende

Cesium Chloride structure:

r  0.170 Cs   0.939 r 0.181 Cl

 cubic sites preferred

So each Cs+ has 8 neighboring Cl-

Adapted from Fig. 12.3, Callister 7e.

Chapter 12 - 13 AX Crystal Structures

Zinc Blende structure r 2 0.074 Zn   0.529  O ?? r 0.140 H O2

• Size arguments predict Zn2+

in OH sites, • In observed structure Zn2+

in TD sites 2+ •Why is Zn in TD sites? – bonding hybridization of

zinc favors TD sites

2+ 2- Adapted from Fig. So each Zn has 4 neighboring O 12.4, Callister 7e. Ex: ZnO, ZnS, SiC Chapter 12 - 14 AX2 Crystal Structures structure

Fluorite (CaF2) • cations in cubic sites

•UO2, ThO2, ZrO2, CeO2

• antifluorite structure – cations and anions reversed

Adapted from Fig. 12.5, Callister 7e.

Chapter 12 - 15 ABX3 Crystal Structures •

Ex: complex

BaTiO3

Adapted from Fig. 12.6, Callister 7e.

Chapter 12 - 16 Mechanical Properties

We know that ceramics are more brittle than metals. Why? • Consider method of deformation – slippage along slip planes • in ionic solids this slippage is very difficult • too much energy needed to move one anion past another anion

Chapter 12 - 17 Ceramic Density Computation

Number of formula units/unit cell

n(A  A )   C A VCNA

Volume of unit cell

Chapter 12 - 18 Ceramics Most common elements on earth are Si & O

-4 SiO4 Si4+

O2-

Adapted from Figs. 12.9-10, Callister 7e. crystobalite

•SiO2 (silica) structures are , crystobalite, & • The strong Si-O bond to a strong, high melting material (1710ºC)

Chapter 12 - 19 Amorphous Silica

• Silica gels - amorphous SiO2 –Si4+ and O2- not in well-ordered lattice – Charge balanced by H+ (to form OH-) at “dangling” bonds • very high surface area > 200 m2/g

–SiO2 is quite stable, therefore unreactive • makes good catalyst support

Adapted from Fig. 12.11, Callister 7e.

Chapter 12 - 20 Silica

• Dense form of amorphous silica – Charge imbalance corrected with “counter cations” such as Na+ – Borosilicate glass is the pyrex glass used in labs • better temperature stability & less brittle than sodium glass

Chapter 12 - 21

4- – Combine SiO4 tetrahedra by having them share corners, edges, or faces

Adapted from Fig. 12.12, Callister 7e.

Mg2SiO4 Ca2MgSi2O7 – Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding

Chapter 12 - 22 Layered Silicates

• Layered silicates (clay silicates)

–SiO4 tetrahedra connected together to form 2-D plane

•(SiO )2- Adapted from Fig. 2 5 12.13, Callister 7e. • So need cations to balance charge =

Chapter 12 - 23 Layered Silicates 2- • clay Al2(Si2O5)(OH)4alternates (Si2O5) layer 2+ with Al2(OH)4 layer

Adapted from Fig. 12.14, Callister 7e.

Note: these sheets loosely bound by van der Waal’s forces Chapter 12 - 24 Layered Silicates

• Can change the counterions – this changes layer spacing – the layers also allow absorption of

•Micas KAl3Si3O10(OH)2.雲母

Mg3(Si2O5)2(OH)2 滑石

Chapter 12 - 25 Forms

• Carbon black – amorphous – surface area ca. 1000 m2/g • – tetrahedral carbon • hard – no good slip planes • brittle – can cut it – large – jewelry – small diamonds • often man made - used for cutting tools and polishing Adapted from Fig. – diamond films 12.15, Callister 7e. • hard surface coat – tools, medical devices, etc.

Chapter 12 - 26 Carbon Forms -

• layer structure – aromatic layers

Adapted from Fig. 12.17, Callister 7e.

– weak van der Waal’s forces between layers – planes slide easily, good lubricant

Chapter 12 - 27 Carbon Forms – Fullerenes and Nanotubes • Fullerenes or carbon nanotubes – wrap the graphite sheet by curving into ball or tube – Buckminister fullerenes(20hexagon+12pentagon)

• Like a soccer ball C60 -also C70 + others

Adapted from Figs. 12.18 & 12.19, Callister 7e.

Chapter 12 - 28 Defects in Ceramic Structures

• Frenkel Defect --a cation is out of place. • Shottky Defect --a paired set of cation and anion vacancies.

Shottky Defect: Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 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.) Frenkel Defect

Q / kT • Equilibrium concentration of defects ~ e D

Chapter 12 - 29 Impurities

• Impurities must also satisfy charge balance = Electroneutrality • Ex: NaCl Na+ Cl- cation • Substitutional cation impurity vacancy Ca2+ Na+ Na+ Ca2+ initial geometry Ca2+ impurity resulting geometry • Substitutional anion impurity anion vacancy O2-

Cl- Cl- initial geometry O2- impurity resulting geometry Chapter 12 - 30 Ceramic Phase Diagrams

MgO-Al2O3 diagram:

Adapted from Fig. 12.25, Callister 7e.

Chapter 12 - 31 Measuring Elastic Modulus

• Room T behavior is usually elastic, with brittle failure. • 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. F cross section L/2 L/2 Adapted from Fig. 12.32, Callister 7e. d R b d = midpoint rect. circ. deflection • Determine elastic modulus according to: F F L3 F L3 x E = = F  4bd3  12R4 slope =  rect. circ.  cross cross linear-elastic behavior section section Chapter 12 - 32 Measuring Strength

• 3-point bend test to measure room T strength.

cross section F L/2 L/2 Adapted from Fig. 12.32, d R Callister 7e. b d = midpoint rect. circ. deflection location of max tension

• Flexural strength: • Typ. values: Material fs(MPa) E(GPa) 1.5Ff L Ff L fs   Si nitride 250-1000 304 bd 2 R3 Si carbide 100-820 345 F rect. Al oxide 275-700 393 F f x glass (soda) 69 69 Data from Table 12.5, Callister 7e.  fs Chapter 12 - 33 Measuring Elevated T Response

• Elevated Temperature Tensile Test (T > 0.4 Tm).

test   x . slope = ss = steady-state creep rate  time

Chapter 12 - 34 Summary

• Ceramic materials have covalent & ionic 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 is brittle, with negligible deformation. • Elevated T creep properties are generally superior to those of metals (and ).

Chapter 12 - 35