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Engineering Materials

ENGG ZC232

Karthik Chethan V. and Ramesh Babu A.

BITS Pilani, Hyderabad Campus

2014

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Outline

Introduction to materials and atomic forces

(different bonds such as metallic, ionic, covalent,secondary etc.).

Crystallography and structures of variousmaterials (metals, ceramics and polymers).

X-ray diffraction (principle, instrumentation,materials characterization, indexing anddetermination of structures).

Structural defects (imperfections).

Phase transformations and Phase diagrams. Mechanical properties (stimulus and response)

(various modes of testing-tensile, compressiveetc.) (short-term, long-term, static and dynamic).

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No strategies.

Attend lectures as much as possible. Be consistent.

Be conceptual and compartmentalizeissues.

Interact thoroughly with instructor and fellow students(if possible).

Question and question some more and come up withhypothetical examples and case studies withschematics (you will be surprised).

Get comfortable, clear doubts and alleviate your fear.

Follow a real, physical, evidence-based problem-solving approach.

Do not be unrealistic. 4

Useful Guidelines

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MaterialsInvestigatingmaterials andrelationshipsthat exist between the structure

and properties of materials (chemistry and

physics). Metals, ceramics, polymers, semi-conductors,

pharmaceuticals, biomaterials (skin, bone,

tissue) etc.

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Engineering Materials

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Engineering Materials

Engineeringon the basis of these structure-

property correlations, designing or

engineering STRUCTURES (bridge), DEVICES

(sensor) or MACHINES (printing machine,

windmill) of a material to produce a pre-

determined set of properties.

Optical, electrical, mechanical, thermal,magnetic etc.

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Atomic Structure and Bonding

Internal Structure - Atomic structure

Bonding

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Atomic Structure and Bonding

Sub atomicelectrons and nuclei (protons

and neutrons) (do not forget quantum

mechanics)

Atomicorganization of atoms or molecules

(crystal structure and clustering of the same)

Microscopicgroups of atoms or molecules

that are normally agglomerated together

Macroscopicviewable with the naked eye

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Structure

What are they (composition)

How heavy and how much space do they occupy(mass and volume)

Physical appearance (size, shape and color) How and Why are they arranged (arrangementpattern and type of bonding)

How and Why do they move (dynamics)

How do they eventually interact How do they respond to various stimuli

How is all of this related to properties

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Structure

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Aluminum oxide may be transparent, translucent, or

opaque depending on the material structure.

single crystal

polycrystal:

low porosity

polycrystal:

high porosity

OPTICAL

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Types of Materials

Metals: strong, ductile, tough, high density,

conductors.

Ceramics: strong, brittle, low density,

insulators.

Polymers: moderately strong, ductile, low

density, insulators.

Composites: strong, ductile, low density,

conductors, insulators.

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John Dalton (1766-1844) found the evidence

of those "hooks in his quantitative chemical

measurements, making the foundation of

modern atomic theory of matter.

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Bonds in Materials

Metals: valence electrons are detached from atoms, andspread in an 'electron sea' that "glues" the ions together.Strong, ductile, conduct electricity and heat well, are shinyif polished.

Ceramics: atoms behave like either positive or negativeions, and are bound by Coulomb forces in addition tocovalent bonding. They are usually combinations of metalsor 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 weakvan der Waals forces, and usually based on C and H. Theydecompose at moderate temperatures (100400 C), andare lightweight. Examples: plastics and rubber.

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Structure of Atom

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Structure of Atoms

The bonding mechanisms between atoms are closelyrelated to the structure of the atoms themselves.

Atoms = nucleus (protons and neutrons) + electrons

Charges:

Electrons and protons have negative and positive chargesof the same magnitude, 1.6 10-19 Coulombs.

Neutrons are electrically neutral.

Masses:

Protons and Neutrons have the same mass, 1.67 10-27 kg. Mass of an electron is much smaller, 9.11 10-31 kg and

can be neglected in calculation of atomic mass.

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Chapter 2 - 18

Electronic Structure

Electrons have wavelike and particulateproperties.

This means that electrons are in orbitalsdefined by a

probability.

Each orbital at discrete energy level determined byquantum numbers.

Quantum # Designation

n= principal (energy level-shell) K, L, M, N, O (1, 2, 3, etc.)

l= subsidiary (orbitals) s,p, d, f (0, 1, 2, 3,, n-1)

ml= magnetic 1, 3, 5, 7 (-lto +l)

ms= spin , -

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Chapter 2 - 19

Electron Energy States

1s

2s2p

K-shell n = 1

L-shell n = 2

3s3p M-shell n = 3

3d

4s

4p4d

Energy

N-shell n = 4

have discrete energy states tend to occupy lowest available energy state.

Electrons...

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Chapter 2 - 20

Electron Configurations

Valence electronsthose in unfilled shells

Filled shells more stable

Valence electrons are most available forbonding and tend to control the chemical

properties

example: C (atomic number = 6)

1s2

2s2

2p2

valence electrons

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Types of Bonding

The electronic structure of atoms defines the

character of their interaction among each

other. Filled outer shells result in a stable

configuration as in noble inert gases. Atomswith incomplete outer shells strive to reach

this noble gas configuration by sharing or

transferring electrons among each other formaximal stability. Strong primary bonding

results from the electron sharing or transfer.

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Primary bonding: electrons are transferred or shared

Strong (100-1000 KJ/mol or 1-10 eV/atom)

Metallic: the atoms are ionized, loosing some electrons

from the valence band. Those electrons form aelectron sea, which binds the charged nuclei in place.

Ionic: Strong Coulomb interaction among negativeatoms (have an extra electron each) and positive atoms(lost an electron). Example - Na+Cl-

Covalent: electrons are shared between the molecules,to saturate the valency. Example - H2

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Types of Bonding-Primary

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Types of Bonding-Secondary

Secondary Bonding: no e- transferred or shared

Interaction of atomic/molecular dipoles

Weak (< 100 KJ/mol or < 1 eV/atom)

Fluctuating Induced Dipole Permanent dipole bonds (polar molecules - H2O, HCl...)

Polar molecule-induced dipole bonds (a polar moleculeinduces a dipole in a nearby nonpolar atom/molecule)

Dipole-dipole Dipole-ion

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Primary

Metallic Bonding Valence electrons are detached from atoms, and spread in

an 'electron sea' that "glues" the positive ions together.

A metallic bond is non-directional (bonds form in anydirection) atoms pack closely. The bonds do notbreak when atoms are rearranged metals canexperience a significant degree of plastic deformation(WHY).

Examples of typical metallic bonding: Cu, Al, Au, Ag, etc.

Transition metals (Fe, Ni, etc.) form mixed bonds that are

comprising of metallic bonds and covalent bonds involvingtheir 3d-electrons. As a result the transition metals aremore brittle (less ductile) that Au or Cu.

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Metallic Bonding

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Ionic Bonding

Ionic Bonding is typical for elements that are situated at thehorizontal extremities of the periodic table. Atoms from the left(metals) are ready to give up their valence electrons to the (non-metallic) atoms from the right that are happy to get one or a fewelectrons to acquire stable or noble gas electron configuration.

As a result of this transfer mutual ionization occurs: atom that givesup electron(s) becomes positively charged ion (cation), atom thataccepts electron(s) becomes negatively charged ion (anion).

Formation of ionic bond:

Ions are attracted by strong coulombic interaction

Oppositely charged atoms attract each other

An ionic bond is non-directional (ions may be attracted to oneanother in any direction)

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Chapter 2 -

Occurs between + and - ions.

Requires electron transfer. Large difference in electronegativity required.

Example: NaCl

Ionic Bonding

Na (metal)unstable

Cl (nonmetal)unstable

electron

+ -CoulombicAttraction

Na (cation)stable

Cl (anion)stable

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A strong electrostatic attraction between positivelycharged Na+ ions and negatively charged Cl- atomsalong with Na+ - Na+ and Cl- - Cl- repulsion result in theNaCl crystal structure which is arranged so that each

sodium ion is surrounded by Cl- ions and each Na+ ionis surrounded by Cl- ions.

Any mechanical force that tries to disturb the electricalbalance in an ionic crystal meets strong resistance:ionic materials are strong and brittle. In some special

cases, however, significant plastic deformation can beobserved, e.g. NaCl single crystals can be bent by handin water.

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Chapter 2 -

C: has 4 valence e-,

needs 4 more

H: has 1 valence e-,

needs 1 more

Electronegativitiesare comparable.

Covalent Bonding similar electronegativityshare electrons

bonds determined by valences&porbitalsdominate bonding

Example: CH4shared electronsfrom carbon atom

shared electrons

from hydrogenatoms

H

H

H

H

C

CH4

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Secondary Bonding

Secondary = van der Waals = physical (as opposite to chemical bondingthat involves e- transfer) bonding results from interaction of atomic ormolecular dipoles and is weak, ~0.1 eV/atom or ~10 kJ/mol.

Permanent dipole moments exist in some molecules (called polarmolecules) due to the asymmetrical arrangement of positively andnegatively regions (HCl, H2O). Bonds between adjacent polar molecules

permanent dipole bondsare the strongest among secondary bonds.

Polar molecules can induce dipoles in adjacent non-polar molecules andbond is formed due to the attraction between the permanent and induceddipoles.

Even in electrically symmetric molecules/atoms an electric dipole can be

created by fluctuations of electron density distribution. Fluctuatingelectric field in one atom A is felt by the electrons of an adjacent atom,and induce a dipole momentum in this atom. This bond due to fluctuatinginduced dipoles is the weakest (inert gases, H2, Cl2).

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Chapter 2 -

Arises from interaction between dipoles

Permanent dipoles-molecule induced

Fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

SECONDARY BONDING

asymmetric electronclouds

+ - + -

secondarybonding

HH HH

H2 H2

secondarybonding

ex: liquid H2

H Cl H Clsecondarybonding

secondarybonding

+ - + -

secondary bonding

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Hydrogen Bonding (different phases of

ice/water)

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Bond Energies

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Chapter 2 -

TypeIonicCovalent

Metallic

Secondary

Bond EnergyLarge!Variablelarge-Diamond

small-BismuthVariablelarge-Tungstensmall-Mercurysmallest

CommentsNondirectional (ceramics)

Directional

(semiconductors, ceramicspolymer chains)

Nondirectional (metals)

Directional

inter-chain (polymer)

inter-molecular

Summary: Bonding

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Bond Energies (potential energy-distance

and attraction between atoms)

This is typical potential well for two interacting atoms. The repulsion betweenatoms, when they are brought close to each other, is related to the Pauli principle:when the electronic clouds surrounding the atoms starts to overlap, the energy ofthe system increases abruptly.

The origin of the attractive part, dominating at large distances, depends on theparticular type of bonding (depth and shape of the well, bond lengths and unitcells).

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Bond Force

Forces can be calculated from the potential

energy of interatomic interaction. For

example, for a system of two atoms (e.g. a

diatomic molecule), the potential dependsonly on the distance between the two atoms

U(r12)

Electron volt or joules is used

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Bond Energy Expression

The net potential energy between two adjacent

atoms or ions, EN, may be represented by,

EN = EA(attractive potential) + ER

(repulsive potential)

EN = - A/r + B/rn

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Chapter 2 - 41

Bond length, r

Bond energy, Eo

Melting Temperature, Tm

Tmis larger if Eois larger.

Properties From Bonding: Tm

ro r

Energy

r

larger Tm

smaller Tm

Eo =

bond energy

Energy

ro r

unstretched length

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Chapter 2 - 42

Coefficient of thermal expansion, a

a~ symmetry at ro

ais larger if Eois smaller.

Properties From Bonding : a

= a (T2-T1)DL

Lo

coeff. thermal expansion

DL

length, Lounheated, T1

heated, T2

ro r

larger a

smaller a

Energy

unstretched length

E

oE

o

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Chapter 2 - 43

Ceramics

(Ionic & covalent bonding):

Metals

(Metallic bonding):

Polymers(Covalent & Secondary):

Large bond energylarge T

mlarge E

small aVariable bond energy

moderate Tmmoderate Emoderate a

Directional PropertiesSecondary bonding dominates

small Tm

small E

large a

Summary: Primary Bonds

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