1. JIF 419
Materials Science
Course Manager: Assoc Prof Dr Saw Kim Guan
Textbook: Materials Science and Engineering (4th
Ed) by Callister & Rethwisch
2. Academic Planner
• 2 assignments: 30 Nov 2015 (1st assignment)
15 Feb 2016 (2nd assignment)
• Web-conference sessions: 3 before Intensive
• Intensive course (19 Jan 2016 - 7 Feb 2016)
• Continuous Examination: types of materials, atomic
structure, bonding in solids, crystal structures,
crystallographic points, directions and planes, point defects
• The contributions of course work and examinations:
Final Exam: 70%
Assignments: 10%
Continuous exam: 20%
2
3. Classification of materials
(metals)
• Atoms in metals are very orderly and dense
• Metallic materials have large numbers of non-
localized electrons (electrons NOT bound to
particular atoms)
• These delocalized electrons make metals to be
good electrical & heat conductor and NOT
transparent to visible light.
4. Classification of materials
(ceramics)
• Ceramics are compounds b/w metallic & non-
metallic elements
• Mostly oxides, nitrides and carbides
• e.g. aluminium oxide, silicon dioxide, silicon
carbide, silicon nitride, porcelain
• Hard, brittle, strong, able to stand heat
• Modern usage – engine parts, cookware,
cutlery
5. Classification of materials
(polymers)
• Many polymers are organic compounds that
are chemically based on carbon, hydrogen and
non-metallic elements
• Consist of very large molecular structures,
often chainlike and have a backbone of carbon
atoms
• e.g. nylon, polyethylene, silicone rubber
• Ductile and pliable, relatively inert chemically
• Low electrical conductivity and non-magnetic
6. Classification of materials
(composites)
• Composites consists of two or more individual
materials
• Designed to achieve a combination of properties
that is not present in any single material
• designed to incorporate the best characteristics
of each of the component material
• E.g. fibreglass – strong, low density
• Modern usage – sport equipment, engine parts
7. 7
Atomic Structure
• Some of the following properties
1) Electrical
2) Thermal
3) Optical
are determined by electronic structure
8. 8
Electron Configurations
• Valence electrons – electrons in unfilled shells
• Filled shells more stable
• Valence electrons are most available for
bonding and tend to control the chemical
properties
– example: C (atomic number = 6)
1s2 2s2 2p2
valence electrons
9. 9
The Periodic Table• Columns: Similar Valence Structure
Adapted from
Fig. 2.8,
Callister &
Rethwisch 9e.
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
giveup1e-
giveup2e-
giveup3e-
inertgases
accept1e-
accept2e-
O
Se
Te
Po At
I
Br
He
Ne
Ar
Kr
Xe
Rn
F
ClS
Li Be
H
Na Mg
BaCs
RaFr
CaK Sc
SrRb Y
10. 10
• Ranges from 0.9 to 4.1,
Smaller electronegativity Larger electronegativity
• Large values: tendency to acquire electrons.
Electronegativity
11. 11
Each H: has 1 valence e-,
needs 1 more
Electronegativities
are the same.
Fig. 2.12, Callister & Rethwisch 9e.
Covalent Bonding
• similar electronegativity share electrons
• Example: H2
shared 1s electron
from 2nd hydrogen
atom
H
H2
shared 1s electron
from 1st hydrogen
atom
H
12. 12
Covalent Bonding: Carbon sp3
• Example: CH4
C: has 4 valence e-,
needs 4 more
H: has 1 valence e-,
needs 1 more
Electronegativities of C and H
are comparable so electrons
are shared in covalent bonds.
Fig. 2.15, Callister & Rethwisch 9e.
(Adapted from J.E. Brady and F. Senese, Chemistry:
Matter and Its Changes, 4th edition. Reprinted with
permission of John Wiley and Sons, Inc.)
14. 14
• Occurs between + and - ions.
• Requires electron transfer.
• Large difference in electronegativity required.
• Example: NaCl
Ionic Bonding
Na (metal)
unstable
Cl (non-metal)
unstable
electron
+ -
Coulombic
Attraction
Na (cation)
stable
Cl (anion)
stable
15. 15
Primary Bonding
• Metallic Bond -- delocalized as electron cloud
• Ionic-Covalent Mixed Bonding
% ionic character =
where XA & XB are Pauling electronegativities
%)100(x
Ex: MgO XMg = 1.3
XO = 3.5
16. 16
Arises from interaction between dipoles
• Permanent dipoles-molecule induced
• Fluctuating dipoles
-general case:
-ex: liquid HCl
-ex: polymer
Adapted from Fig. 2.20,
Callister & Rethwisch 9e.
Adapted from Fig. 2.22,
Callister & Rethwisch 9e.
Secondary Bonding
asymmetric electron
clouds
+ - + -
secondary
bonding
HH HH
H 2 H 2
secondary
bonding
ex: liquid H 2
H Cl H Clsecondary
bonding
secondary
bonding
+ - + -
secondary bonding
18. 18
• atoms pack in periodic, 3D arrays
Crystalline materials...
-metals
-many ceramics
-some polymers
• atoms have no periodic packing
Noncrystalline materials...
-complex structures
-rapid cooling
crystalline SiO2
noncrystalline SiO2"Amorphous" = Noncrystalline
Adapted from Fig. 3.11(b),
Callister & Rethwisch 9e.
Adapted from Fig. 3.11(a),
Callister & Rethwisch 9e.
Materials and Packing
Si Oxygen
• typical of:
• occurs for:
19. 19
Crystal Systems
7 crystal systems
14 crystal lattices
Unit cell: smallest repetitive volume which
contains the complete lattice pattern of a crystal.
a, b, and c are the lattice constants
21. 21
Crystallographic Directions
1. Determine coordinates of vector tail, pt. 1:
x1, y1, & z1; and vector head, pt. 2: x2, y2, & z2.
2. Tail point coordinates subtracted from head
point coordinates.
3. Normalize coordinate differences in terms
of lattice parameters a, b, and c:
4. Adjust to smallest integer values
5. Enclose in square brackets, no commas
[uvw]
ex:
pt. 1 x1 = 0, y1 = 0, z1 = 0
=> 1, 0, 1/2
=> [ 201 ]
z
x
Algorithm
y
=> 2, 0, 1
pt. 2
head
pt. 1:
tail
pt. 2 x2 = a, y2 = 0, z2 = c/2
22. 22
Crystallographic Directions
-4, 1, 2
families of directions <uvw>
z
x
where the overbar represents a negative
index
[ 412 ]=>
y
Example 2:
pt. 1 x1 = a, y1 = b/2, z1 = 0
pt. 2 x2 = -a, y2 = b, z2 = c
=> -2, 1/2, 1
pt. 2
head
pt. 1:
tail
Multiplying by 2 to eliminate the fraction
23. Crystal Structures
• A lattice is a 3D array of points coinciding with
atomic positions
• The atomic order in crystalline solids indicates
that small groups of atoms form a repetitive
pattern
• The repeat entities are called unit cells
24. Metallic Crystal Structures
• The atomic bonding is metallic and non-
directional in nature
• 3 crystal structures for most common metals –
face-centered cubic structure (FCC), body-
centered cubic structure & hexagonal close-
packed structure
25. Face-centered cubic structure (FCC)
• Has a unit cell of cubic geometry
• Atoms located at each corner and the centers
of all the cubic faces
• The coordination number (the no. of nearest
neighbour atoms) is 12
• The atomic packing factor (sum of the sphere
volumes of all atoms within a unit cell) is 0.74
26. FCC unit cell
Hard sphere unit cell
representation; the unit cell
contains 4 atoms
Reduced-sphere unit cell
representation
27. FCC coordination number
Hard sphere unit cell
representation shows that:
The front face atom X has four
corner nearest neighbour
atoms surrounding it (indicated
as 1, 2, 3, 4), four face atoms
that are in contact from behind
(two of these are indicated as I
and II), and four other face
atoms residing in the next unit
cell to the front, which is not
shown.
28. FCC
Hard sphere unit cell representation
Using Pythagoras’ theorem we have
a2 + a2 = (4R)2
Thus by simplifying we have
a (unit cell length) = 2R 2 = 2.83R
29. FCC unit cell volume
Hard sphere unit cell representation
V= a3 = (2R 2)3 =8×2×R3 2=16 R3 2
30. Atomic packing factor for FCC
The atomic packing factor is the fraction of the
solid sphere volume in a unit cell
APF = vol of atoms in unit cell/total unit cell vol
Note that there are 4 atoms per FCC unit cell
Vol of atoms = 4 × =
Total unit cell vol = 16 R3 2
34
3
R 316
3
R
APF = = 0.74
3
3
16
3
16 2
R
R
31. Body centered cubic structure
• The BCC structure has a cubic unit cell with
atoms located at all eight corners and one
atom at the cube center
• The single atom at the center is wholly
contained within the unit cell
• The coordination number is 8
• The atomic packing factor is 0.68
32. BCC unit cell
Hard sphere unit cell
representation; the unit cell
contains 2 atoms
Reduced-sphere unit cell
representation
The unit cell length a is 2.31 R
33. BCC coordination number
Hard sphere unit cell
representation shows that:
Each center atom X has the
eight corner atoms as it nearest
neighbours (indicated by 1, 2,
…7. The last atom 8 is hidden
behind X and not shown)
Therefore the coordination no.
is 8 for BCC structure
34. 34
Atomic Packing Factor: BCC
APF =
4
3
π ( 3 a/4 ) 3
2
atoms
unit cell atom
volume
a 3
unit cell
volume
length = 4R =
Close-packed directions:
3 a
• APF for a body-centered cubic structure = 0.68
a
RAdapted from
Fig. 4.1(a), Callister &
Rethwisch 9e.
a
a2
a3
