Ceramics Term ceramics comes from the greek word keramikos – “burnt stuff” Ceramics are...

Ceramics• Term ceramics comes from the greek word keramikos –

“burnt stuff”

• Ceramics are typically formed during high temperature heat treating – “Firing”

• Traditionally ceramics included:• China• Porcelain• Bricks (both construction and refractory)• Tiles• Glasses

• Over the last 60 years or so…there has been an explosion in new technologies similar to other areas of material science

• Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity.

• Amount of ionic bond character:

Ceramic Bonding

SiC: small

CaF2: large

Eq 2.10: % ionic character = {1 – exp[-(0.25)(XA – XB)2]} x 100

XA, XB are electronegativities of components A and B

Ionic Ceramics

• Crystal structure are composed of electrically charged ions– Cations (Fe3+) – Positive Charge Typically metals

– Anions (O2-) – Negatively Charged Typically non-metals

• Two characteristics influence crystal structure:– The magnitude of charge on the component ions

• Stoichiometry must balance

• Overall charge neutrality is required

– Relative sizes of the component ions anion

cation

r

r small

large

Note that size of ion is affected by charge:

For iron: r(Fe2+) = 0.077 nm, r(Fe3+) = 0.069 nm, r(Fe) = 0.124 nm

Which sites will cations occupy to form stable crystal structure?

Criteria of Site Selection

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. Covalent Bond Hybridization

Ionic Bonding & Structure1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors.

- -

- -+

unstable

• Charge Neutrality: --Net charge in the structure should be zero.

--General form:

- -

- -+

stable

- -

- -+

stable

CaF2: Ca2+cation

F-

F-

anions+

AmXp

m, p determined by charge neutrality

• Coordination # increases with

Coordination # and Ionic Radii

2

rcationranion

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

TD

OH

cubic

ZnS (zincblende)

NaCl(sodium chloride)

CsCl(cesium chloride)

rcationranion

How many anions can you arrange around the cation?

Purely geometrical argument

Geometrical Derivation of Site Size

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

a 2ranion

2ranion 2rcation 2 2ranion

ranion rcation 2ranion

rcation ( 2 1)ranion

2ranion 2rcation 2a

4140anion

cation .r

r

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: We know that an FCC unit cell has 4 OH and 8 TD sites.

If for a specific ceramic each unit cell has 6 cations and the cations prefer OH sites, then

4 in OH

2 in TD

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

%.)XXionic% 511]}exp[-0.25(-{1 100 character 2CSi

• 89% covalent bonding• both Si and C prefer sp3 hybridization

• Therefore in SiC get TD sites

• 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--structure = NaCl

Example: Predicting Structure of FeO

Ionic radius (nm)

0.053

0.077

0.069

0.100

0.140

0.181

0.133

Cation

Anion

Al3+

Fe2+

Fe3+

Ca2+

O2-

Cl-

F-

Rock Salt StructureSame concepts can be applied to ionic solids in general Example: NaCl (rock salt) structure

rNa = 0.102 nm

rNa/rCl = 0.564

cations prefer OH sites

rCl = 0.181 nm

AX Crystal Structure: equal number of Anion and Cation locations

MgO and FeOMgO and FeO also have the Rock Salt structure

O2- rO = 0.140 nm

Mg2+ rMg = 0.072 nm

rMg/rO = 0.514

cations prefer OH sites

So each oxygen has 6 neighboring Mg2+

2nd Type of AX Crystal Structure

939.0181.0

170.0

Cl

Cs

r

r

Cesium Chloride structure:

cubic sites preferred

So each Cs+ has 8 neighboring Cl-

3rd Type of AX Crystal Structures

So each Zn2+ has 4 neighboring S2-

Zinc Blende structure?? 529.0

140.0

074.0

2

2

O

ZnHO

r

r

• Size arguments predict Zn2+ in OH sites, • In observed structure Zn2+ in TD sites

• Why is Zn2+ in TD sites?– bonding hybridization of

zinc favors TD sites

Ex: ZnO, ZnS, SiC

AX2 Crystal StructuresFluorite structure

• Calcium Fluorite (CaF2)

• Cations in cubic sites

• UO2, ThO2, ZrO2, CeO2

antifluorite structure –

rC/rA for CaF2 is about 0.8 – coordination number of 8 cubic structure

But, stoichiometry calls for ½ as many Ca2+ as F- ions 8 cubes in unit cell

cations and anions reversed

ABX3 Crystal StructuresPerovskite crystal structure

Ex: Barium Titanate – BaTiO3

Temperatures above 120oF – cubic crystal structure

Summary of Common Structures

Close Packing of Anions

Since Anions are commonly packed in FCC structure – we can talk about close packed planes of anions

Coordination = 4 Coordination = 6

Can have both:FCC Stacking – ABCABCHCP Stacking – ABABAB

Cl- form FCC LatticeClose packed planes are {111}

Mechanical Properties

Why are ceramics more brittle than metals?

• Consider method of deformation– In metals we have dislocation motion along slip

planes– Slip planes are the close packed planes

• In ionic solids dislocation motion is very difficult– Why? Too much energy needed to move one anion

past another anion