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MCE 230Materials Science

Fall 2010

Lecture - 4

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Atomic Structure

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Bonding Forces

Two types of bonding forces:

1. Attractive (FA ): particular type of bonding between twoatoms

2. Repulsive (FR ): forces come from the interaction between

electrons in the outer shells

Both are function of the Interatomic distance (separation)

The net for FN = FA + FR

When FA = FR then FN = 0 (equilibrium state at r0

(equilibrium spacing) )

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Potential Energies

One can use potential energies instead

Bonding energy (E0 ) : the potential energy required to separatethe two atoms to an infinite separation ( potential energy at

equilibrium (at r0)

Many material properties depend on; the magnitude of (E0 ), the

shape of energy shape and the type of bonding (melting point,

modulus of elasticity, thermal expansion, etc..)

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Types of Bonding

Primary bonding: electrons are transferred or shared Secondary bonding: interaction of atomic dipoles ( no

electrons are transferred or shared)

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Primary Interatomic Bond

Three different types of primary bond are found in solid:

1. Ionic

2. Covalent

3. Metallic

The nature of each bond depends on the electron

structure of its atoms

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Ionic Bond Between metallic and nonmetallic elements

Electron transfer

Between +ve andve ions

Characteristics: hard, brittle, electrically and thermally

insulative , and high melting point.

Na (metal)

unstable

Cl (nonmetal)

unstable

electron

+ -Coulombic

Attraction

Na (cation)

stable

Cl (anion)

stable

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Covalent Bond Adjacent atoms share electrons ( to reach stable state) en

Examples: H2, F2, CH4, H2O, diamond (carbon), silicon. Possible number of covalent bonds = Capacity of outer

shellnumber of valence electrons

Characteristics: may be very strong and has a highmelting temperature (diamond), and may be weak and

has low melting temperature.

It is possible to have Interatomic bonds that are partially

ionic and partially covalent

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CH4

Carbon

has 4 valence electrons

and needs 4 more to be

stable

Hydrogen

Has 1 valence electrons

and needs 1 more to be

stable

Covalent Bond

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Metallic Bond The valence electrons of

each atoms form an electroncloud which is belonging to

the entire metal (all atoms )

(ve in charge)

The remaining nonvalence

electrons and nuclei are

called ion cores ( +ve in

charge )

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It is the primary bond in metals and their alloys Characteristics: bonding may be weak or strong,

good heat and electricity conductors

Metallic Bond

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Very weak compare to the primary bonds

Usually they exists between all atoms, but when one

of the primary bonding exists the effect of secondary

bond is negligible.

It is due to interactions between dipoles ( a dipole

exists whenever there is some separation of positive

and negative portions within an atom or a molecule)

Secondary Bonds

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1. Fluctuating induced dipole bonds

Instantaneous and short-live distortions of the electrical

symmetry of atoms or molecules due to vibrational motion

So small dipoles are formed

Secondary Bonds

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2. Polar molecule induced dipole bonds

Polar molecule has permanent dipole because of anasymmetrical arrangement of positive and negative regions

within this molecule.

Polar molecule may induce dipoles in an adjacent nonpolarmolecule to form a polar molecule induced dipole

This bond is stronger than the fluctuating one

Secondary Bonds

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Bonding: Summary

Ionic: bond energy is largeCovalent: bond energy varies

Metallic: bond energy varies

Secondary: bond energy is the smallest

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Bonding: Summary

Ceramics: Covalent & Ionic Bonding

Examples: Silicon Carbide (SiC) Large melting temperature (> 3000 C)

Large stiffness (E = 400 GPa)

Small thermal expansion coefficient: 4.4x10-6 C-1

Metal: Metallic Bonding Example: carbon steel

Moderate melting temperature (1300 C)

Moderate Stiffness (E = 207 GPa)

Moderate Thermal expansion coefficient 11.5x10-6

C-1

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Bonding: Summary

Polymers: Secondary Bonding Example: Polyethylene (HD)

Low melting temperature (137 C)

Low stiffness (E = 1.08 GPa)

High thermal expansion coefficient (150x10-6 C-1)

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Crystalline Structure

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Structure of Solids

The structure of the material is the way its atomsare arranged.

Types of solid structure:Crystalline: where atoms are arranged in a repeated or

periodic way ( metals, some polymer and manyceramics)

Noncrystalline: (amorphous): ( some polymers and

some ceramics)

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Unit cell

Unit Cell: the smallest arrangement of atoms thatis repeated (basic structural unit or buildingblock)

The unit cell defines the structure through:

GeometryAtoms positioning within the unit cell

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Unit cell Atoms Positioning

Number of whole atoms belonging to the unit cell

Coordination number: number of touching neighboratoms

Atomic Packing Factor (APF): how much the unit cellvolume is filled with atoms

Relation between the atomic radius and the unit celldimension

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Metallic Structures FCC: Face Centered Cubic

Aluminum, Copper, Gold, Nickel

BCC: Body Centered Cubic Iron, Chromium, Tungsten

HCP: Hexagonal Closed Packed

Titanium, Zinc, Cobalt