1.0) PHYSICAL METALLURGY

• LECTURE OUTLINE:- Primary Bonding (atomic bonding).- Define crystal lattice.- Define the methods for measuring distance

between atom through.(Assignment 2)- Disfigurement in crystal structure.

Primary Bonding• Ionic• Covalent• Metallic

2

• Bonding involves the valence electrons.• Bonding occurs due to the tendency of the

atoms to assume stable electron structures (completely filled outer shells)

3

• Occurs between + and - ions.

• Requires electron transfer.

• Large difference in electronegativity required.

• Example: NaCl

Ionic Bonding

Na (metal)

unstable

Cl (nonmetal)

unstable

electron

+ - Coulombic Attraction

Na (cation)

stableCl (anion)

stable

4

• Predominant bonding in Ceramics

Adapted from Fig. 2.7, Callister & Rethwisch 3e. (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

Give up electrons Acquire electrons

NaCl

MgO

CaF 2

CsCl

6

C: has 4 valence e-, needs 4 more

H: has 1 valence e-, needs 1 more

Electronegativities are comparable.

Covalent Bonding• similar electronegativity share electrons

• bonds determined by valence – s & p orbitals dominate bonding

• Example: polymers, GaAs, InSb, SiC, CH4

shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH 4

Metallic Bonding

•Metallic bonds have up to 3 valence electrons that are not bound to a specific atom.•They drift throughout the metal forming a “sea of electrons” or “electron cloud”. •The nonvalence electrons and nuclei for the “ion cores”.•The free electrons act as a “glue” to hold the ion cores together.•These are good conductors of heat and charge (electricity).

Secondary Bonding (van der Waals)

• Interaction between dipoles; dipoles are a separation of charge (+/-).

• Weaker forces (10kJ/mol) than primary bonding, yet these bonds still influence physical properties.

• Secondary bonding exists in virtually all atoms and molecules, but their presence may be obscured by primary bonding.

8

9

• Permanent dipoles-molecule induced

• Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

SECONDARY BONDINGasymmetric electron

clouds

+ - + -secondary

bonding

HH HH

H 2 H 2

secondary bonding

ex: liquid H 2

H Cl H Clsecondary bonding

secondary bonding

+ - + -

secondary bondingsecondary bonding

10

TypeIonicCoulombic force

Covalent

Metallic

SecondaryVan der Waals

Bond EnergyLarge

Variable

large-Diamond

small-Bismuth

Variable

large-Tungsten

small-Mercury

smallest

CommentsNondirectional (ceramics)

Directional(semiconductors, ceramicspolymer chains)

Nondirectional (metals)

Directionalinter-chain (polymer)inter-molecular

Summary: Bonding

11

Ceramics

(Ionic & covalent bonding):

Large bond energylarge Tm

large Esmall

Metals

(Metallic bonding):

Variable bond energymoderate Tm

moderate Emoderate

Summary: Primary Bonds

Polymers

(Covalent & Secondary):

Directional PropertiesSecondary bonding dominates

small Tm

small E large

secondary bonding

Crystal Structure

Crystal Structure

Crystal Structure

Crystal Structure

Crystal Structure

Crystal Structure

Crystal Structure

Crystal Structure

Disfigurement Crystal Structure

Edge Dislocation

Screw Dislocation

Screw Dislocation

Point Defect

Thank You