Atomic Bonding & Material Properties...Page 1 UNIVERSITY OF NAIROBI Lecture 1 cont.. Atomic Bonding...

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Lecture 1 cont..

Atomic Bonding & Material Properties

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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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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.

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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

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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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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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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.


12

12

oL

LTT

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•

~ 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

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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.

https://en.wikipedia.org/wiki/Gecko

https://en.wikipedia.org/wiki/Spatulae_%28biology%29

https://en.wikipedia.org/wiki/Van_der_Waals_force

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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

http://en.wikipedia.org/wiki/Ballistic_vest

http://en.wikipedia.org/wiki/Ballistic_vest

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Lecture -Evaluation

1. Explain bonding and structure in ceramics

2. Explain general properties of ceramics & their applications

Atomic Bonding & Material Properties...Page 1 UNIVERSITY OF NAIROBI Lecture 1 cont.. Atomic Bonding...

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