Material Science-B.tech 2012
Transcript of Material Science-B.tech 2012
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Material Science
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Objective and scope of the present course
Primary objective is to present the basic fundamentals of materials
science and engineering.
Expose the reader community to different classes of materials, their
properties, structures and imperfections present in them.
Help understand the subject with ease by presenting the content in asimplified and logical sequence at a level appropriate for
students/teachers/researchers.
Aid the teaching learning process through relevant illustrations,
animations, web content and practical examples.
Highlight important concepts for each topic covered in the subject
Provide opportunity of self-evaluation on the understanding of the
subject matter.
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Materials in Day to day life
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What is materials science ?
Material science is the investigation of the relationship among
processing, structure, properties, and performance of materials.
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Introduction
Historical Perspective
Stone → Bronze → Iron → Advanced materials
What is Materials Science and Engineering ?
Processing → Structure → Properties → Performance
Classification of Materials
Metals, Ceramics, Polymers, Semiconductors
Advanced Materials
Electronic materials, superconductors, etc.
Modern Material's Needs, Material of Future
Biodegradable materials, Nanomaterials, “Smart” materials
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Historical Perspective
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A better understanding of structure-composition properties relations has lead to a
remarkable progress in properties of materials. Example is the dramatic progress in
the strength to density ratio of materials, that resulted in a wide variety of new
products, from dental materials to tennis racquets.
M. A. White, Properties of Materials (Oxford University Press, 1999)
Stone age to IT age
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300,000 BC
200,000 BC
Stone age
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5000 BC
5000 BC
4000 BC
3500 BC
Introducing metals
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1450 BC
1500 AD
1855 AD
20th
Century
Iron Age
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The golden era
1886 AD
1890-1910
AD
1939
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The electronic revolution
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throughout the Iron Age many new types of materials
have been introduced
ceramic, semiconductors, polymers, composites…
Age of advanced materials
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Metals: valence electrons are detached from atoms, and spread in an
'electron sea' that "glues" the ions together. Strong, ductile, conduct
electricity and heat well, are shiny if polished.
Semiconductors: the bonding is covalent (electrons are shared between
atoms). Their electrical properties depend strongly on minute proportions
of contaminants. Examples: Si, Ge, GaAs.
Ceramics: atoms behave like either positive or negative ions, and are
bound by Coulomb forces. They are usually combinations of metals or
semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and
carbides). Hard, brittle, insulators. Examples: glass, porcelain.
Polymers: are bound by covalent forces and also by weak van der Waals
forces, and usually based on C and H. They decompose at moderate
temperatures (100 – 400 C), and are lightweight. Examples: plastics
rubber.
Types of materials
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Materials selection-Properties & cost
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Properties are the way the material responds to the environment
and external forces.
Mechanical properties – response to mechanical forces, strength,etc.
Electrical and magnetic properties - response electrical and
magnetic fields, conductivity, etc.
Thermal properties are related to transmission of heat and heat
capacity.
Optical properties include to absorption, transmission andscattering of light.
Chemical stability in contact with the environment corrosion
resistance.
Properties
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Subatomic level
Electronic structure of individual atoms that
defines interaction among atoms (inter atomicbonding).
• Atomic level
Arrangement of atoms in materials (for the
same atoms can have different properties, e.g.two forms of carbon: graphite and diamond)
• Microscopic structure
Arrangement of small grains of material that can
be identified by microscopy.
• Macroscopic structure
Structural elements that may be viewed with the
naked eye.
Structure
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Aluminum
Does structure changes material?
Glass
Rubber
f l
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Design of materials having specific desired characteristics directly from our
knowledge of atomic structure.
Miniaturization: “Nanostructured" materials, with microstructure that haslength scales between 1 and 100 nm with unusual properties.
Electronic components, materials for quantum computing.
Smart materials: airplane wings that adjust to the air flow conditions,
buildings that stabilize themselves in earthquakes…
Environment-friendly materials: biodegradable or photodegradable plastics,
advances in nuclear waste processing, etc.
Learning from Nature: shells and biological hard tissue can be as strong as the
most advanced laboratory-produced ceramics, mollusces produce biocompatible
adhesives that we do not know how to reproduce…
Materials for lightweight batteries with high storage densities, for turbine
blades that can operate at 2500°C, room-temperature superconductors? chemical
sensors (artificial nose) of extremely high sensitivity, cotton shirts that never
require ironing…
Future of material science
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What is materials science ?
Material science is the investigation of the relationship among
processing, structure, properties, and performance of materials.
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The bonding mechanisms between atoms are closely related to the
structure of the atoms themselves.Atoms = nucleus (protons and neutrons) + electrons
Charges: Electrons and protons have negative and positive charges
of the same magnitude, 1.6 × 10-19Coulombs.
Neutrons are electrically neutral.
Masses: Protons and Neutrons have the same mass, 1.67 × 10-27kg.
Mass of an electron is much smaller, 9.11 × 10-31kg and can be neglected
in calculation of atomic mass.
The atomic mass (A) = mass of protons + mass of neutrons
# protons gives chemical identification of the element # protons =
atomic number (Z) # neutrons defines isotope number
Structure of atoms
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Atomic mass unit (amu) = 1/12 mass of Carbon
12 (12C)
1 mol of substance contains 6.023 x 1023
(Avogadro’s number) atoms or molecules.
Atomic weight = 1 amu/atom (or molecule) = 1
g/mol = Wt. of 6.023 x 1023 atoms or molecules.
For example, atomic weight of copper is 63.54
amu/atom or 63.54 g/mole
Structure of atoms
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•When two neutral atoms are brought close to each other,
they experience attractive and or repulsive force
•Attractive force is due to electrostatic attraction between
electrons of one atom and the nucleus of the other.
Atomic Bonding
Atomic interaction
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Repulsive force arises due to repulsion between electrons
and nuclei of the atoms.
The net force, FN (Fig. a), acting on the atoms is the
summation of attractive and repulsive forces.
The distance, at which the attraction and repulsion forces
are equal and the net force is zero, is the equilibrium
interatomic distance, ro. The atoms have lowest energy at
this position.
Attraction is predominant above ro and repulsion is
dominant below ro (see Fig. a).
Atomic interaction
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Atomic interaction
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is the distance at which the interaction energy is zero.is the depth of the potential well (see Fig. b) and is a
measure of the bonding energy between two atoms.
L-J potential can be also expressed in the simplified form
as VLJ = A/r12 – B/r6 and hence, is also known as 6-12
potential.
A/r12 is predominant at short distances and hence,
represents the short-range repulsive potential due tooverlap of electron orbitals and –B/r6 is dominant at longer
distance and hence, is the long range attractive potential.
Atomic interaction
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The mechanisms of bonding between the atoms are
based on the foregoing discussion on electrostatic interatomicinteraction.
The types of bond and bond strength are determined by
the electronic structures of the atoms involved.
The valence electrons take part in bonding. The atoms
involved acquire, loose or share valence electrons to
achieve the lowest energy or stable configuration of noble
gases.
Atomic bonding can be broadly classified as i) primary
bonding ii) secondary bonding
Atomic Bonding
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Primary bonds
Three types primary bonds are found in solids
Metallic
Ionic
Covalent
Majority of the engineering materials consist of one
of these bonds. Many properties of the materials
depend on the specific kind of bond and the bondenergy
Atomic Bonding
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Ionic bonds are generally found in compounds composed of
metal and non-metal and arise out of electrostatic attractionbetween oppositely charged atoms (ions).
Number of electron in outer shell is 1 in Na and 7 in Cl .
Therefore, Na will tend to reject one electron to get stable
configuration of Ne and Cl will accept one electron to obtainAr configuration. The columbic attraction between Na+ and
Cl¯ions thus formed will make an ionic bond to produce NaCl.
Some other examples are CaF2, CsCl , MgO, Al2O3.
Ionic Bond
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In this type of bonding, atoms share their valence electronsto get a stable configuration.
Methane (CH4): Four hydrogen atoms share their valence
electrons with one carbon atom and the carbon atom in
turn shares one valence electron with each of the four
hydrogen atoms. In the process both H and C atoms get
stable configuration and form a covalent bond.
Covalent Bond
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In metals the valence electrons are not really bound to oneparticular atom, instead they form a sea or cloud of valence
electrons which are shared by all the atoms. The remaining
electrons and the nuclei form what is called the ion core
which is positively charged. The metallic bond arises out of
the columbic attraction between these two oppositely
charged species – the electron cloud and the ion cores.
Metallic Bond
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Ionic and covalent bonds posses high bond energy –450
– 1000 kJ/mole
High bond strength in ionic and covalent solids results inhigh melting point, high strength and hardness. e.g.
diamond
As the electrons are tightly bound to the atoms they are
generally poor conductors of heat and electricity
Are brittle in nature
Characteristics of primary bonds
Structure-property correlation
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Metallic bonds on the other hand provide good thermal and
electrical conductivities as the valence electrons are free tomove.
The metallic bond energy is 68 kJ/mol (Hg) on the lower
side and 850 kJ/mol (W, tungsten) on the higher side.
Bond strength increases with atomic number as more
electrons are available to form the bonds with the ion cores.
As a result melting point, hardness and strength increases
with atomic number.
Metals are ductile as the free moving electrons provides
agility to the bonds and allows plastic deformation.
Structure-property correlation
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Diffusion Phenomena
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Diffusion Mechanism
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Kirkendall Effect
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Kirkendall Effect
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Fick’s lawSteady-state diffusion
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Fick’s second lawNon- Steady-state diffusion