Metal Structure Atoms held together by metallic bonding - - PowerPoint PPT Presentation

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

48

Metal Structure

• Atoms held together by metallic bonding
• Crystalline structures in the solid state, almost

without exception

• BCC, FCC, or HCP unit cells

– Body-centered cubic (BCC)

• Chromium, Iron, Molybdenum, Tungsten

– Face-centered cubic (FCC)

• Aluminum, Copper, Gold, Lead, Silver, Nickel

– Hexagonal close-packed (HCP)

• Magnesium, Titanium, Zinc

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

49

Ceramic Structure

• Most ceramics have crystal structure, while

glass (SiO2) is amorphous

• Molecules characterized by ionic or covalent

bonding, or both

• % ionic character increases with difference in

electronegativity

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

50

Ceramic Bonding

He

• Ne
• Ar
• Kr
• Xe
• Rn
• 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 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

C 2.5 Si 1.8

F 4.0

Ca 1.0

Table of Electronegativities CaF2: large SiC: small

• Large vs small ionic bond character:

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

51

Ionic Bonding & Structure

• Charge Neutrality:
• -Net charge in the

structure should be zero.

• -General form: AmXp

m, p determined by charge neutrality

• Stable structures:
• -maximize the # of nearest oppositely charged neighbors.
• +

unstable

• +

stable

• +

stable

CaF2:

Ca2+ cation F- F- anions

+

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

52

Coordination # & Ionic Radii

• Coordination # increases with

Issue: How many anions can you arrange around a cation?

rcation ranion

rcation ranion Coord # < .155 .155-.225 .225-.414 .414-.732 .732-1.0 ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium chloride) 2 3 4 6 8

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

53

Ex: Predicting Structure of FeO

• On the basis of ionic radii, what crystal structure

would you predict for FeO?

Cation Al3+ Fe2+ Fe3+ Ca2+ Anion O2- Cl- F- Ionic radius (nm) 0.053 0.077 0.069 0.100 0.140 0.181 0.133

• Answer:

rcation ranion = 0.077 0.140 = 0.550

based on this ratio,

• -coord # = 6
• -structure = NaCl

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

54

AmXp Structures

• Consider CaF2 :

rcation ranion = 0.100 0.133 ≈ 0.8

• Based on this ratio, coord # = 8 and structure = CsCl.
• Result: CsCl structure w/only half the cation sites
• ccupied.
• Only half the cation sites

are occupied since #Ca2+ ions = 1/2 # F- ions.

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

55

Polymer Microstructure

• Polymer = many mers

C C C C C C H H H H H H H H H H H H

Polyethylene (PE)

mer Cl Cl Cl C C C C C C H H H H H H H H H

Polyvinyl chloride (PVC)

mer

Polypropylene (PP)

CH3 C C C C C C H H H H H H H H H CH3 CH3 mer

• Covalent chain configurations and strength:

Direction of increasing strength

Adapted from Fig. 14.2, Callister 6e.

Branched Cross-Linked Network Linear

secondarybonding

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

56

• Molecular weight, Mw: Mass of a mole of chains.

smaller Mw larger Mw

• Tensile strength (TS):
• -often increases with Mw.
• -Why? Longer chains are entangled (anchored) better.
• % Crystallinity: % of material that is crystalline.
• -TS and E often increase

with % crystallinity.

• -Annealing causes

crystalline regions to grow. % crystallinity increases.

crystalline region amorphous region

Molecular Weight & Crystallinity

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Material Structure

57

Summary

• Metals are usually crystalline in BCC, FCC or HCP.
• Ceramic materials have mostly covalent & some 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.
• Most polymers are based on carbon and are therefore considered
• rganic chemicals.
• Both amorphous and crystalline structures are possible, although

the tendency to crystallize is much less than for metals or non-glass ceramics.