Post on 22-Dec-2014
description
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