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UNIVERSITY OF
NAIROBI
Lecture 1 cont..
Atomic Bonding & Material Properties
UNIVERSITY OF
NAIROBI
Bonding Forces and Energies
Consider two isolated atoms separated by inter-atomic dist r
At large r, atoms do not interact.
As r gets smaller, an attractive force FA starts to act pulling atoms closer.
As R 0, a repulsive force FR begin to act preventing atoms from getting too close..
r
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UNIVERSITY OF
NAIROBI
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Resultant force is
RANFFF
Repulsive force
Resultant force
Attractive force ro
O
Force (F)
r
At r = ro, FR = FA
and FN = 0
ro is the equilibrium inter-atomic separation dist (ro ≈ 0.3 nm) at which atoms enter into bonding.
UNIVERSITY OF
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FR gives rise to a +ve Potential Energy (VRep) while FA gives rise to a –ve P.E (VAtt) where
o Where , Z1 & Z2 are the Atomic Numbers.
o e = 1.6 x 10-19 C, o = 8.85 x 10-12 F/m
o A, B, and n are constants. n 8.
r
AAtt2
2
21r
RRepdrF V
4drF V anddr
r
eZZr
o
nnmm r
A
rand
r
B
r
1V
1V
AttRep
o
eZZ
4A
2
21
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UNIVERSITY OF
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The net potential
The NET Force
Fig shows variation of VN and FN with r called the Condon-Morse curves
NB. At r = ro, VN = E0 = Bonding energy
E0 = (Potential Energy Well) or min energy required to separate two atoms to an infinite separation.
mnpR
B
R
AV
ReAttNV V
11N F
mn r
mB
r
nA
dr
dV
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r > r0 ; VN increases gradually to 0 as R∞ . The force is attractive
r < r0; VN increases rapidly to ∞ at small separation. The force is repulsive
r R
VN
0 r0
Repulsive
Attractive
r
Eo Potential Energy well
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UNIVERSITY OF
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Force vs. Separation Distance
Energy vs. Separation Distance
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Bonding Energies & Material properties
Material properties depend on
Depth of Energy well, E0
Shape of the P.E well
Type of bonding
The deeper the well, the higher the bonding energy E0, and the stronger the bonding High MP and material exists as solid
Shallow well Low MP and material is gaseous e.g., H2
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UNIVERSITY OF
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MP is larger if Eo is larger.
r o r
Energy
larger MP
smaller MP
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(a) Mechanical properties
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orat curvedistvsForce of slope
dr
dF
Strain
Stress E
Elastic Modulus (E) = measure of resistance to separation of atoms i.e., inter-atomic bonding forces
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the steeper the slope of , the deeper the well, higher the E stronger material
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dr
dF
Smaller E (Weaker material
Large E (Stronger material)
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(b) Thermal properties
Linear thermal expansion coefficient ()
The trough at Eo corresponds to equilibrium inter-atomic spacing at OK.
When a material is heated from T1 to T5, vibrational energy increases thereby increasing the width of the curve.
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12
oL
LTT
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13
•
~ symmetric at ro
is larger if Eo is smaller.
ro
r
smaller
larger
Energy
Eo
Eo
L
length, L o
unheated, T 1
heated, T 2
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Curve is “asymmetric”
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When Eo is small (shallow well), and the curvature is very assymmetric, then, the inter-atomic spacing increase with temp rise indicating high .
is small when Eo is large & the well is deep and narrow
is due to the asymmetric curvature of the P.E trough, rather than the increased atomic vibration amplitudes with rising temp.
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If P.E curve were symmetric, there would be no net change in inter-atomic separation with rise in temp and consequently, no thermal expansion
Metals >> Ceramics >> Polymers Because in metals, the vibrational transfer is through atoms and in ceramics it is through atoms and in polymers, it is due to the rotation and vibration of long chain molecules.
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Activity
Question 1:
a) Explain the thermal expansion of a material on the basis of the P.E -interatomic distance curve.
b) On the same plot sketch the P.E-distance curve of a material with
i) higher thermal expansion. Give example
ii) lower thermal expansion. Give example
c) How can the Young’s modulus be determined from the energy-distance curve?
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Activity -2
Question 2:
Why do ceramics exhibit much lower strength than their theoretically expected strength of E/10?
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+ =
When atoms combine they form compounds that are unique both chemically & physically from its parent atoms.
E.g., Na is a metal that reacts violently with water while Cl is a very poisonous greenish-colored gas
BUT Na + Cl = Salt
Atomic Bonding
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Bonding between the atoms is due to electrostatic interaction between nuclei and electrons.
Atoms enter into bonding to achieve atomic stability determined by Hund’s rule which favours closed electron shelf or half-shells in the atom.
Type of bonding is influenced by the atom’s position in the periodic table
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Periodic Table 7 horizontal rows are called periods.
Elements in a given column or group have similar valence electron structures, as well as chemical and physical properties.
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giv
e u
p 1
e-
giv
e u
p 2
e-
giv
e u
p 3
e-
in
ert
gase
s
acc
ept
1e
-
acc
ept
2e
-
O
Se
Te
Po At
I
Br
He
Ne
Ar
Kr
Xe
Rn
F
Cl S
Li Be
H
Na Mg
Ba Cs
Ra Fr
Ca K Sc
Sr Rb Y
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The Periodic Table • Columns: Similar Valence Structure
Adapted from Fig. 2.6, Callister & Rethwisch 8e.
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
giv
e u
p 1
e-
giv
e u
p 2
e-
giv
e u
p 3
e-
in
ert
gase
s
acc
ept
1e
-
acc
ept
2e
-
O
Se
Te
Po At
I
Br
He
Ne
Ar
Kr
Xe
Rn
F
Cl S
Li Be
H
Na Mg
Ba Cs
Ra Fr
Ca K Sc
Sr Rb Y
UNIVERSITY OF
NAIROBI
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• Ranges from 0.7 to 4.0,
Smaller electronegativity Larger electronegativity
• Large values: tendency to acquire electrons.
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.
Electronegativity
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Types of Atomic & Molecular Bonds
Primary Atomic Bonds
Ionic Bonds
Covalent
Metallic
Secondary Atomic & Molecular Bonds
Permanent Dipole (Van der Waals) bonds
Fluctuating Dipole Bonds
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(a) Ionic Bonding
Occurs between atoms lying at the two extreme ends of the periodic table.
Atoms tend to lose or gain valency electrons to achieve complete outer shells thereby forming ions +ve ions (cations) or -ve ions (anions)
Ionic Bonding results from the electrostatic attractions between +ve and –ve ions
Predominant bonding in Ceramics
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27 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.
Examples: Ionic Bonding
Give up electrons Acquire electrons
NaCl
MgO
CaF2
CsCl
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Ionic Bonding in NaCl
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Properties of ionic bonding
“Nondirectional” - has same strength in all directions ST
cations sorround themselves with as many anions as possible forming a giant molecule
Low electrical & thermal conductivity – No free
electrons. Entire ion must move to conduct electricity
Transparent
Hard and Brittle - because the ions are bound strongly to the
lattice and aren't easily displaced.
High MP and BP - large amt of thermal energy is required to
separate the ions which are bound by strong electrical forces.
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(b) Covalent Bonding
Takes place between atoms with small differences in electronegativity which are close to each other in periodic table (i.e., between non-metals and non-metals lying in the central column of the periodic table ).
Bonding results from sharing of outer s and p electrons so that each atom attains the noble-gas electron configuration.
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Number of ē -pair bonds that an atom can form is determined by the 8-N rule where N = No of the column in the periodic table containing the atom. Thus, F can form 1 bond, O can form 2 bonds etc
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
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Properties of Covalent bonding
Directional ” – strength of bond not equal in all
directions
Low electrical & thermal conductivity – Since
electrons cannot move through the lattice.
Very strong (diamond) or very weak (bismuth).
High MP and BP -because each atom is bound by
strong covalent bonds.
E.g., Diamond, silicon, CH4, H2O, HNO3, H2, Cl2, F2, etc.,
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In metals, all valence electrons in a metal
combine to form a “sea” of electrons that move
freely between the atom cores.
(c ) Metallic Bonding
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A metallic bond results from the electrostatic force of
attraction between +ve ions and delocalized outer
electrons.
The free electrons act as the bond (or as a “glue”)
between the +ve ions. As a result we have a high
ductility (plastic deformation) of metals - the “bonds” do
not “break” when atoms are rearranged.
The more electrons, the stronger the attraction. High
MP and BP and the metal is stronger and harder.
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Properties of Metallic bonding
Non-directional bond
High Thermal & electrical conductivity – Due to free electrons
Ductile, opaque
The metallic bond is weaker than the ionic and the
covalent bonds.
E.g., Na, Cu, Al, Au, Ag, etc.
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NB. Transition metals (Fe, Ni, etc.) form mixed bonds, comprising of metallic and covalent bonds in-volving their 3d-electrons. As a result the transition met-als are more brittle (less ductile) than Au or Cu
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Bond type Example Bond
Energy
Optical
Property
Electrical
Property
Thermal
Property
Mechanical
Property
Ionic NaCl, ZnS
Transparent Semiconductor High MP Hard &
Brittle
Covalent Diamond,
Graphite
Transparent
& Coloured
Insulators V. High MP
& BP
V. Hard
Metallic Na, Fe, Cu,
Ag
Opaque &
Reflecting
Conductors Good heat
conductors
Tough &
Ductile
Molecular
( Van der
Waals)
Ne, Ar, Xe,
Phenol,
Transparent Insulators Low MP Soft and
brittle
Hydrogen
Bonding
Ice, Organic
solids, H2,
CH4
Transparent Insulators Low MP Soft and
brittle
incr
ease
s
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Activity
Explain the general properties of ionic, covalent and metallic bonding giving examples in each case
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Secondary Bonds (Van der Waal)
They are “physical bonds” involving no electron movement
Secondary bonds are as a result of the interaction of the electric dipoles contained in atoms or molecules
A dipole exists in a molecule if there is asymmetry in its electron density distribution due to large difference in electronegativities between atoms, S.T. there is some separation of positive and negative portions of an atom or molecule.
Special case: Hydrogen bonding.
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Can be divided by:
(1) Fluctuating Dipoles
(2) Permanent Dipoles
Fluctuating dipoles are due asymmetrical electron charge distribution within the atoms that changes in both direction and magnitude with time.
symmetric asymmetric
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Idealized symmetrical electron charge cloud distribution
Real case with “asymmetrical” electron charge cloud distribution that changes with time, creating a Fluctuating electric dipoles
E.g
Electron charge cloud distribution in a noble-gas atom
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Permanent Dipoles
Polar Molecules have Permanent dipole and can induce dipoles in adjacent non-polar molecules and bonding can take place between the permanent and induced dipoles.
E.g. Hydrogen bonding
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Examples of Hydrogen Bonding:
o HF,
o HCl
o H2O,
o Polymers
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In hydrogen bonding, the H end of the molecule is positively charged and can bond to the negative side of another H2O molecule (the O side of the H2O dipole)
“Hydrogen bond” – secondary bond formed between two permanent dipoles in adjacent water molecules.
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The bigger a molecule is, the easier it is to
polarise (to form a dipole), and so the van
der Waal's forces get stronger, so bigger
molecules exist as liquids or solids rather
than gases. Physical Bonds (no electron
involvement).
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The ability of geckos – to hang on vertical or upside down on flat surface has been attributed to the van der Waals forces between these surfaces and the spatulae on their toes.
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Questions How can the high electrical and thermal conductivities of metals be explained by the “electron gas” model of metallic bonding? Ductility?
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SOLUTION
The high electrical and thermal conductivities of metals are explained by the mobility of their outer valence electrons in the presence of an electrical potential or thermal gradient.
The ductility of metals is explained by the bonding “electron gas” which enables atoms to pass over each other during deformation, without severing their bonds.
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Summary
A deep and narrow trough in the curve indicates large bond energy, high MP, large elastic modulus and small
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Lecture -Evaluation
1. Explain ionic, covalent and metallic bonding
2. Explain secondary bonding and differentiate between permanent and fluctuating induced dipole bonds giving examples of each
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General Properties of Materials
•
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Metals
Composed of one or more metallic elements e.g., Iron, Copper, Aluminum.
Have crystalline structure with metallic bonding
Valence electrons are detached from atoms, and spread in an 'electron sea' that "glues" the ions
together.
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Metals and Alloys
Ferrous
Eg: Steel,
Cast Iron
Nonferrous
Eg:Copper
Aluminum
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General Properties
Strong in Tension & ductile with high fracture toughness
Good conductors of electricity & heat
Reflective (Shinny if polished) and Opaque to light
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Ceramics
• Properties & applications
• Classification
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“Ceramics” means burnt stuff properties achieved through high-temperature heat treatment (firing).
Ceramics are inorganic, non-metallic materials i.e., a combinations of metals or semiconductors with oxygen, nitrogen or carbon (e.g., Al2O3, NaCl,
SiC, SiO2)
Typically produced using clays and other minerals or chemically processed powders
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Bonding and structure
bonds are mixture of ionic & covalent i.e., atoms behave like +ve or –ve ions, and are bound by Coulomb forces.
Type of bonding results in either crystalline (with atoms arranged in regular repetitive pattern) or amorphous (non-crystalline) e.g., glass
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SEM of ceramic showing mullite crystals – Amorphous
crystalline
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Diversity in properties ( Mechanical, Optical, Thermal, Electrical and Magnetic properties) stems from type internal structure and bonding
Material properties are influenced by microstructural features viz:
grain size
Porosity & secondary phases
grain boundaries
Imperfections such as
micro-cracks, defects
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e.g. Elastic modulus of ceramics decreases with increase in Porosity
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Depence of Flexural strength (MOR) on porosity
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0
5
10
15
20
25
5 10 15 20
Volume Porosity (%)
Fle
xura
l Str
ength
(M
Pa)
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General Properties
Brittle with low fracture Toughness
Extreme hardness & wear resistant - Everlasting !!!
Corrosion resistant
Heat resistance
Low Thermal Conductivity
Low Electrical Conductivity
High heat capacity (high MP upto
1,600°C )
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Wide range of applications
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Insulation in brick walls
Thermal insulators
Applications Thermal insulator
Abrasives
Construction materials
Cookery
Examples - Porcelain, Glass, Silicon nitride.
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Classification of Ceramics
Classified according to major functions i.e. Bonded Clay (“Traditional”) ceramics & Advanced ceramics
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Classification of Ceramics (a) Bonded “Traditional” ceramics
Are Clay-based porous ceramics They include
These include:
(a) Structural Clay Products
pottery, porcelain, tiles & Whitewares (Wall tiles, Electrical porcelain & Decorative ceramics)
Bricks
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(b) Refractory Ceramics
High temp applications
(d) Cement, glass
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Advanced Ceramics
Exhibits superior mechanical, electrical, optical, properties and corrosion or oxidation resistance.
Classified according to: Oxides: alumina, zirconia,
Have low thermal conductivity & Used as thermal barriers to protect metals surfaces from wearing out
Non-oxide ceramics: carbides and nitrides -SiC, Si3N4 etc.
Extremly hard & used as polishing tools
Composites: reinforced materials for high toughness e.g., bioceramics
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zirconia
SiC – polishing tools
Ceramic Matrix Composite (CMC) rotor
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Bioceramic implants Silicon carbide is used for inner plates of ballistic vests
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Lecture -Evaluation
1. Explain bonding and structure in ceramics
2. Explain general properties of ceramics & their applications