material science 2 ppt

7/29/2019 Material Science 2 PPT

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Atomic structure Important properties of the solid material depend on the

geometrical atomic arrangement and also the interaction betweenatoms or molecules

Each atom consist of nucleus composed of protons and neutrons whichis encircled by moving electrons. The top most orbital electrons, valance electrons affect most material

properties that are of interest to engineer. E.g. chemical properties, nature of bonding size of an atom

Both proton and electron are electrically charged with magnitude of

1.6*10^-19 C. Masses are infinitesimally small Mp & Mn =~ 1.67*10^-27Kg it is also called AMU(atomic mass unit) Me=~ 9.11*10^-31Kg Atomic Number(Z)_ Number of protons in the nucleus

for electrically neutral atom the atomic number can also be equal tonumber of electrons. 1


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Atomic mass(A)_.

It Is the sum of masses of proton and neutron inside thenucleus

Atomic mass is always measured in atomic mass unit(amu) A=~Z+N


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Atomic bonding in solids what makes atom to cluster together.? How atoms are arranged.?

Inter atomic bonds Primary bonds Secondary bonds

Primary bonds

they are relatively stronger &exists in almost all solids Ionic Covalant Metallic

Secondary bonds Relatively weaker bonds. Exists in many substances like water alongwith primary bonds

Van der walls Hydrogen


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Metallic Bonds Metals are characterized by high thermal and electrical conductivities.

This bond comes into picture when a valance electrons are shared

between number of atoms. Unlike covalent bond shared electrons are not specific pair of atoms i.e.

electrons are delocalized

Since the shared electrons are delocalized the metallic bonds are nondirectional

High thermal and electrical conductivity is the result of thisdelocalization.


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Covalent bond This bond comes into existence if valence electrons are shared

between a pair of atoms, thus acquire stability by saturating thevalence configuration.

Covalent bonds are stereospecific i.e. each bond is between aspecific pair of atoms, which share a pair of electrons

Covalent bonds are very strong and directional in nature

The diamond is the result of covalent bond where the carbon atom

is covalently bonded with 4 neighbor atoms and each neighboratom is bonded with equal number of atoms to form a rigidstructure

Red= electron from hydrogen

blue= electron from carbon


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Ionic Bonds Exists between 2 atoms when 1 is +ve and other is ve charged.

+ve charged==loss of electron

-ve charged==extra electron Strong direct attraction

They are non directional in nature

E.g. NaCl

Na is +ve and Cl is ve

They are the strongestbonds

Usually exists in material

along with the covalant

bond

.


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Secondary bonds Van der waals bond

Origin is quantum mechanical in nature & mainly due to the dipole

moment of atoms or molecules Usually atoms are neutral and hence no dipole moment

Polarized atoms experience the van der waal forces

Weak in nature

E.g. carbon tetrocloride

Hydrogen bond

Exists between the atoms/molecules having permanent dipole moment

E.g. Water

e- in oxygen tens to concentrate away from hydrogen so the resulting charge

difference between molecule allows the bonding of water molecule. More strong than vander waal forces

Important for life


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Crystal It is a material which constitute atoms,molecules, or ions which are

arranged in orderly pattern extending in all 3 dimensions

Structures which are not crystalline are called as amorphousstructures.


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Crystal structure

Arrangement of crystals in specific order/pattern to form a3D structure is called as crystal structure.

To analyze arrangement of atom we use 2 basic technique Lattice and basis concept

Unit cell

ASSUMPTION_ atoms as a hard sphere with fix radius

Lattice and basis

lattice_ its an arrangement of points such that surroundings ofeach point in the lattice are identical

Types_ 1D 2D 3D

Basis_ group of 1 or more atoms located in particular way withrespect to each other and associated with each lattice point isknown as basis.

Crystal structure Atoms of basis placed on lattice point.


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Possible ways of arranging point in lattice structure 1D__ only 1 way 1 1D lattice

2D__ 5 ways 5 2D lattice structures

3D__ 14 ways

14 3D lattice structuresTo arrange a point in such a way that so as to have the same surrounding

Known as Bravias lattices

These 14 structures are grouped into 7 crystal systems

Cubic_____________________ cube

Tetragonal_________________ square prism Orthorhombic______________ rectangular prism

Hexagonal_________________ 120 degree rhombic prism

Trigonal___________________ parallelepiped

Monoclinic_________________ parallogramic prism

Triclinic____________________ parallelepiped


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Based on the lattice point in the unit cell we have 4types of Bravais lattice Simple cubic, Face centered, Body centered , Base centered

Atoms are arranged at the corner ofthe cube.

Face centered_ atoms are arranged at the

corner and at the each face

of the cube.


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Body centered_Atoms are arranged at each

corner and at the center ofthe cube

Base centered_Atoms are arranged at each corner of thecube and 1 atom on each top and bottomplane of hexagon.


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Sr.No. Crystalsystem

shape Bravais lattice

SC FC BC Base C.

1 cubic Cube Y Y Y

2 Tetragonal Square prism Y Y

3 Orthorhombic Rect. Prism Y Y Y Y

4 Hexagonal 120 degreerhombic prism

Y

5 Trigonal Parallelepiped Y

6 Monoclinic Parallogramicprism

Y Y

7 Triclinic parallelepiped Y


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https://www.e-education.psu.edu/matse201/node/8

U i ll


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Unit cell Main aim was to analyze the structure of the atom and to develop the

notion to model the crystalline solid structures

Unit cell concept is different from lattice and basis but the end result

is Representation of Crystal Structure_ Definition_

Single unit which when duplicated and translated, reproduces theentire crystal structure is called as unit cell.

E.g 2D crystal Rubiks cube


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Concept of unit cell and 2D structure In 2D structure we have (1/4) atom per corner

With 4 corners (1/4)*4=1 unit atom per unit cell

In 3D structurewe have 8 corners and (1/8) atom at eachcorner

For simple cubic structure_

Base Upper layer


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For body centered cubic structure


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For face centered cubic structure


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Miller Indices_ Introduced by British mineralogist Willam mille in 1839.

They are defined w.r.t. any unit cell

Any crystal consist of an infinite planes seperated by perticulardistance a

Miller indices are nothing but the numbers used to designate the palnein a crystal

As well as it gives the direction of the plane

Pr dur t find mill r indi


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Procedure to find miller indices Consider a right handed co-ordinate system

Determine the co-ordinates that lie on the required direction

Subtract the co-ordinates of tail point from the head point to

obtained the number of lattice parameters traveled in thatparticular direction

Clear fractions

Enclose the number in square bracket []

If negative sign is produced then represent negative sign with barover the number.

E.g


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material science 2 ppt

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