keramik 1

Chapter 12 -

ATOMIC BONDING IN

CERAMIC

1

Chapter 12 -

2

Chapter 12 -

Figure 1.7 Periodic table with ceramic compounds indicated by a combination of one or more

metallic elements (in light color) with one or more nonmetallic elements (in dark color). Note

that elements silicon (Si) and germanium (Ge) are included with the metals in this figure but

were not included in the periodic table shown in Figure 1.4. They are included here because,

in elemental form, Si and Ge behave as semiconductors (Figure 1.16). Elemental tin (Sn) can

be either a metal or a semiconductor, depending on its crystalline structure.

Ceramics are usually oxides. However, silicon nitride (Si3N4) is an important nonoxide ceramic

used in a variety of structural applications. Some ceramics are chemical compounds made up

of one of the five nonmetallic materials, C, N, O, P or S, shaded with dark blue color in figure

1.7. Very many variety of ceramic materials can be formed.

(C, N, P, S are forming none-oxide ceramics with metallic elements.) (Now, Si and Ge are included as metallic elements in this classification, because they form ceramics.)

Nonmetallic ceramic forming

elements Metallic Elements

Chapter 12 - 4

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

-- % ionic character increases with difference in

electronegativity of atoms.

Adapted from Fig. 2.7, Callister & Rethwisch 8e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the

Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by

Cornell University.)

Degree of ionic character may be large or small:

Atomic Bonding in Ceramics

SiC: small

CaF2: large

Chapter 12 - 8

Occurs between + and - ions.

Requires electron transfer.

Large difference in electronegativity required.

Example: NaCl

IONIC BONDING

Chapter 12 -

Characteristics of Ionic Bonding

1. medium high melting point (600 -

2000 C)

2. medium high boiling points

3. hard and brittle

4. nonconductor of electricity

5. poor conductor of heat

Chapter 12 - 9

Predominant bonding in Ceramics

Give up electrons Acquire electrons

He -

Ne -

Ar -

Kr -

Xe -

Rn -

F 4.0

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Ca 1.0

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

CsCl

MgO

CaF2

NaCl

O 3.5

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 by Cornell University.

EXAMPLES: IONIC BONDING

Chapter 12 - 10

Requires shared electrons

Example: CH4

C: has 4 valence e,

needs 4 more

H: has 1 valence e,

needs 1 more

Electronegativities

are comparable.

Adapted from Fig. 2.10, Callister 6e.

COVALENT BONDING

Chapter 12 -

Characteristics of Covalent Bonding

1. very low melting point (-370 to 300 C)

2. very low boiling point

3. soft

4. nonconductor of electricity

5. poor conductor of heat

Chapter 12 - 11

Molecules with nonmetals

Molecules with metals and nonmetals

Elemental solids (RHS of Periodic Table)

Compound solids (about column IVA)

He -

Ne -

Ar -

Kr -

Xe -

Rn -

F 4.0

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Ca 1.0

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

SiC

C(diamond)

H2O

C 2.5

H2

Cl2

F2

Si 1.8

Ga 1.6

GaAs

Ge 1.8

O 2.0

co

lum

n I

VA

Sn 1.8

Pb 1.8

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 by Cornell University.

EXAMPLES: COVALENT BONDING

Chapter 12 - 11

Ceramic Phase Diagrams

MgO-Al2O3 diagram:

Adapted from Fig.

12.25, Callister &

Rethwisch 8e.

Chapter 12 - 12

fig_12_26

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CERAMIC CRYSTAL STRUCTURES

Chapter 12 - 14

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

Chapter 12 - 15

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

Adapted from Fig. 12.1,

Callister & Rethwisch 8e.

- -

- - +

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

Chapter 12 - 16

Coordination # increases with

Coordination # and Ionic Radii

Adapted from Table 12.2,


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

(zinc blende)

NaCl (sodium

chloride)

CsCl (cesium chloride)

r cation r anion

To form a stable structure, how many anions can

surround around a cation?

Chapter 12 - 17

Chapter 12 - 18

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

Chapter 12 - 19

Chapter 12 - 20

Chapter 12 - 21

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

Chapter 12 - 22

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

Data from Table 12.3,


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 -

Chapter 12 - 23

Rock Salt Structure

Same concepts can be applied to ionic solids in general.

Example: NaCl (rock salt) structure

rNa = 0.102 nm

rNa/rCl = 0.564

cations (Na+) prefer octahedral sites



rCl = 0.181 nm

Chapter 12 - 24

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

EXAMPLE OF CRYSTAL STRUCTURE

Rock salt structure(AX)(NaCl ) Fluorite structure(AX2)(CaF2)

Perovskite structure(ABX3)(BaTiO3) Spinel structure(AB2X4)(MgAl2O4)

16.03.2015

25 http://www.eng.uwo.ca/es021/ES021b_2007/Lecture%20Notes/Chap%2012-13%20SN%20-%20Ceramics.pdf

EXAMPLE OF CRYSTAL STRUCTURE

16.03.2015

Chapter 12 - 27

AX Crystal Structures

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-

AXType Crystal Structures include NaCl, CsCl, and zinc blende

Chapter 12 - 28

AX2 Crystal Structures

Calcium Fluorite (CaF2)

Cations in cubic sites

UO2, ThO2, ZrO2, CeO2

Antifluorite structure

positions of cations and

anions reversed



Fluorite structure

Chapter 12 - 29

ABX3 Crystal Structures



Perovskite structure

Ex: complex oxide

BaTiO3

Chapter 12 -

VMSE: Ceramic Crystal Structures

30

Chapter 12 - 31

Density Computations for Ceramics

A

AC )(

NV

AAn

C

=

Number of formula units/unit cell

Volume of unit cell

Avogadros number

= sum of atomic weights of all anions in formula unit

AA

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

Chapter 12 - 32

Chapter 12 - 33

Chapter 12 - 34

Silicate Ceramics Most common elements on earth are Si & O

SiO2 (silica) polymorphic forms are quartz, crystobalite, & tridymite

The strong Si-O bonds lead to a high melting temperature (1710C) for this material

Si4+

O2-

Adapted from Figs.

12.9-10, Callister &

Rethwisch 8e crystobalite

Chapter 12 - 35

Bonding of adjacent SiO44- accomplished by the

sharing of common corners, edges, or faces

Silicates

Mg2SiO4 Ca2MgSi2O7

Adapted from Fig.

12.12, Callister &

Rethwisch 8e.

Presence of cations such as Ca2+, Mg2+, & Al3+

1. maintain charge neutrality, and

2. ionically bond SiO44- to one another

Chapter 12 - 36

Chapter 12 - 37

Quartz is crystalline SiO2:

Basic Unit: Glass is noncrystalline (amorphous) Fused silica is SiO2 to which no impurities have been added

Other common glasses contain impurity ions such as Na+, Ca2+,

Al3+, and B3+

(soda glass)



Glass Structure

Si0 4 tetrahedron 4-

Si 4+

O 2 -

Si 4+ Na +

O 2 -

Chapter 12 - 38

Layered Silicates Layered silicates (e.g., clays, mica, talc)

SiO4 tetrahedra connected together to form 2-D plane

A net negative charge is associated with each (Si2O5)

2- unit

Negative charge balanced by adjacent plane rich in positively charged cations

Adapted from Fig.

12.13, Callister &

Rethwisch 8e.

Chapter 12 - 39

Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)4

2+

layer

Layered Silicates (cont.)

Note: Adjacent sheets of this type are loosely bound to one another by van der Waals forces.



Chapter 12 - 40

Polymorphic Forms of Carbon

Diamond tetrahedral bonding of

carbon hardest material known

very high thermal

conductivity

large single crystals gem stones

small crystals used to grind/cut other materials

diamond thin films hard surface coatings

used for cutting tools, medical devices, etc.



Chapter 12 - 41

Polymorphic Forms of Carbon (cont)

Graphite layered structure parallel hexagonal arrays of

carbon atoms

weak van der Waals forces between layers planes slide easily over one another -- good

lubricant

Adapted from Fig.

12.17, Callister &

Rethwisch 8e.

Chapter 12 - 42

Polymorphic Forms of Carbon (cont) Fullerenes and Nanotubes

Fullerenes spherical cluster of 60 carbon atoms, C60 Like a soccer ball

Carbon nanotubes sheet of graphite rolled into a tube

Ends capped with fullerene hemispheres

Adapted from Figs.

12.18 & 12.19, Callister

& Rethwisch 8e.

Chapter 12 - 43

Electroneutrality (charge balance) must be maintained

when impurities are present

Ex: NaCl

Imperfections in Ceramics

Na + Cl -

Substitutional cation impurity

without impurity Ca 2+ impurity with impurity

Ca 2+

Na +

Na + Ca 2+

cation vacancy

Substitutional anion impurity

without impurity O 2- impurity

O 2-

Cl -

an ion vacancy

Cl -

with impurity

Chapter 12 - 44

Vacancies

-- vacancies exist in ceramics for both cations and anions

Interstitials -- interstitials exist for cations

-- interstitials are not normally observed for anions because anions are large relative to the interstitial sites

Adapted from Fig. 12.20, Callister

& Rethwisch 8e. (Fig. 12.20 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.)

Point Defects in Ceramics (i)

Cation

Interstitial

Cation

Vacancy

Anion

Vacancy

Chapter 12 - 45

Frenkel Defect -- a cation vacancy-cation interstitial pair.

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

Equilibrium concentration of defects

Adapted from Fig.12.21, Callister

& Rethwisch 8e. (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.)

Point Defects in Ceramics (ii)

Shottky

Defect:

Frenkel

Defect

/kTQDe

Chapter 12 - 46

Chapter 12 - 47

Chapter 12 - 48

Chapter 12 - 49

keramik 1

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covalent bonding chapter

large chapter

nacl ionic bonding chapter

atomic bonding

predominant bonding

nonmetallic elements

nonmetallic ceramic

poor conductor of heat