Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 1

Chapter 12: Structures & Properties of

Ceramics

ISSUES TO ADDRESS...

• How do the crystal structures of ceramic materials

differ from those for metals?

• How do point defects in ceramics differ from those

defects found in metals?

• How are impurities accommodated in the ceramic lattice?

• How are the mechanical properties of ceramics

measured, and how do they differ from those for metals?

• In what ways are ceramic phase diagrams different from

phase diagrams for metals?

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 2

• Bonding: -- Can be ionic and/or covalent in character.

-- % ionic character increases with difference in

electronegativity of atoms.

• Degree of ionic character may be large or small:

Atomic Bonding in Ceramics

SiC: small

CaF2: large

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 3

Ceramic Crystal Structures

Oxide structures – oxygen anions larger than metal cations

– close packed oxygen in a lattice (usually FCC)

– cations fit into interstitial sites among oxygen ions

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 4

Factors that Determine Crystal Structure 1. Relative sizes of ions – Formation of stable structures: --maximize the # of oppositely charged ion neighbors.

- -

- - +

unstable

- -

- - +

stable

- -

- - +

stable

2. Maintenance of

Charge Neutrality : --Net charge in ceramic

should be zero.

--Reflected in chemical

formula:

CaF 2 : Ca 2+

cation

F -

F -

anions +

A m X p

m, p values to achieve charge neutrality

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 5

• Coordination # increases with

Coordination # and Ionic Radii

2

r cation r anion

Coord

#

< 0.155

0.155 - 0.225

0.225 - 0.414

0.414 - 0.732

0.732 - 1.0

3

4

6

8

linear

triangular

tetrahedral

octahedral

cubic

ZnS

NaCl

CsCl

r cation r anion

To form a stable structure, how many anions can surround

around a cation? Coordination number

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 6

Computation of Minimum Cation-Anion

Radius Ratio • Determine minimum rcation/ranion for an octahedral site

(C.N. = 6)

a = 2ranion

2ranion 2rcation= 2 2ranion

ranion rcation= 2ranion

rcation= ( 2 1)ranion

arr 222 cationanion =

414.012anion

cation ==r

r

//upload.wikimedia.org/wikipedia/commons/1/14/Octahedron.gif

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 7

Bond Hybridization

Bond Hybridization is possible when there is significant

covalent bonding

– hybrid electron orbitals form

– For example for SiC

• XSi = 1.8 and XC = 2.5

% ionic character = 100 {1- exp[-0.25(XSi XC)2]} =11.5%

• ~ 89% covalent bonding

• Both Si and C prefer sp3 hybridization

• Therefore, for SiC, Si atoms occupy tetrahedral sites

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 8

• On the basis of ionic radii, what crystal structure

would you predict for FeO?

• Answer:

5500

1400

0770

anion

cation

.

.

.

r

r

=

=

based on this ratio,

-- coord # = 6 because

0.414 < 0.550 < 0.732

-- crystal structure is NaCl

Example Problem: Predicting the Crystal Structure of FeO

Ionic radius (nm)

0.053

0.077

0.069

0.100

0.140

0.181

0.133

Cation

Anion

Al 3+

Fe 2 +

Fe 3+

Ca 2+

O 2-

Cl -

F -

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 9

AX-Type Crystal Structures: Zinc Blende Structure

• ZnS (zinc blende or sphalerite) structure

rZn = 0.075 nm

rZn/rS = 0.408

cations (Zn2+) prefer tetrahedral sites

rS = 0.184 nm

So each Zn2+ has 4 neighbor S2-

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 10

AX-Type Crystal Structures: Rock Salt Structure

• NaCl (rock salt) structure

rNa = 0.102 nm

rNa/rCl = 0.564

cations (Na+) prefer octahedral sites

rCl = 0.181 nm

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 11

MgO and FeO

O2- rO = 0.140 nm

Mg2+ rMg = 0.072 nm

rMg/rO = 0.514

cations prefer octahedral sites

So each Mg2+ (or Fe2+) has 6 neighbor oxygen atoms

MgO and FeO also have the NaCl structure

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 12

939.0181.0

170.0

Cl

Cs ==

r

r

Cesium Chloride structure:

Since 0.732 < 0.939 < 1.0,

cubic sites preferred

So each Cs+ has 8 neighbor Cl-

AX-Type Crystal Structures: CsCl Structure

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 13

AmXp Crystal Structures: AX2

• Calcium Fluorite (CaF2)

• Cations in cubic sites

• UO2, ThO2, ZrO2, CeO2

• Antifluorite structure –

positions of cations and

anions reversed

Fluorite structure

75201330

1000.

.

.==

F

Ca

r

r

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 14

AmBnXp Crystal Structures: ABX3

• Perovskite structure

Ex: complex oxide

BaTiO3

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 15

AmBnXp Crystal Structures: AB2X4

• Spinel structure:

Ex: MgAl2O4

Mg2+ in tetrahedral sites

Al3+ in octahedral sites

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 16

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 17

Density Computations for Ceramics

A

AC )(

NV

AAn

C

=

Number of formula units/unit cell

Volume of unit cell

Avogadro’s number

= sum of atomic weights of all anions in formula unit

AA

AC = sum of atomic weights of all cations in formula unit

Prof. Yo-Sep Min Fusion Technology of Chemical & Materials Engineering Lecture 21, Chapter 12 - 18

ANNOUNCEMENTS

Reading: pp. 464 ~ 491