Download Materials Science for Chemical Engineers

CBE4010 Introduction to Materials Science and Engineering for Chemical Engineers
K. - K. Koo
1. INTRODUCTION
Material
- substance intended for some practical application
- generally a SOLID
Materials Science
-
investigating the relationships that exist between the STRUCTURE,
PROPERTIES and PROCESSING/SYNTHESIS of materials
Materials Engineering
- on the basis of structure-property relationships, design or engineering the
structure of a material to produce pre-determined properties.
- interdisciplinary: Metallurgy, Ceramic Eng., Chemical Eng., Mechanical Eng.,
Electrical Eng., Solid State Physics, Physical Chem., etc..
Structure-Property-Processing Concept
PROPERTIES


STRUCTURE  PROCESSING
Properties - microscopic or bulk response to external stimuli,
i.e., force vs. deformation: strength, ductility
- mechanical, electrical, thermal, magnetic, optical, deteriorative
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Structure - subatomic
- atomic: arrangement of atoms in a crystal
- microscopic: size, shape and orientation of crystals
- macroscopic
Processing - shaping, treating conditioning etc. to alter form, structure and properties
mechanically, thermally or chemically
(composition/microstructure) - performance - synthesis/processing concept
(structure-property-processing concept)
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Classification of materials: based partly on chemical make-up, partly on atomic
structure and bonding and partly on application.
- Metals and Alloys
- Ceramics, Glasses, and Glass-ceramics
- Polymers ( plastics )
- Composite Materials
- Electronic Materials ( Semiconductors )
- Biomaterials
Metals
- large number of nonlocalized electrons.
- good electrical and thermal conductivity, opaqueness, reflectivity,
high strength with some ductility.
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Ceramics
- compounds between metals and nonmetals ( C, N, O, P, S )
- oxides( Al2O3, MgO, SiO2...), nitrides( Si3N4, … ), carbides( SiC, … )
- minerals, clays, glasses, cements
- insulative to electricity and heat, more resistant to harsh environments,
hard, but brittle.
- crystalline, noncrystalline(glass)
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Polymers
- plastics and rubbers: orgnic compounds of H, C and other nonmetals
(O, N, F, Si)
- large molecular structures
- low density, electrically and thermally insulating often flexible
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Composites - engineered materials consisting of mixture of different primary materials
- glass/resin, graphite/epoxy, ceramic/metal, polymer/metal
- wood, concrete
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Semiconductors - semiconducting materials
- elemental semiconductor ( Si, Ge, Sn, ….)
- compound semiconductor ( GaAs, CdS, ZnO, ….)
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Environmental and Other Effects on Properties of Materials
The structure-property relationships in materials are often influenced by the
surroundings to which the material is subjected during use: temperature, corrosion or
oxidation, fatigue, sudden impact.
Materials Design and Selection
Selection of material having the needed properties and the potential to be
manufacture economically and safely into a useful product is a complicated process
requiring the knowledge of the structure-property-composition relationships.
Summary
______________________________________________________________
Properties of materials (mechanical, electrical, optical, magnetic, thermal, ...)

a detail knowledge of atomic structure

the electron structure of the atoms
the binding force the atoms
the three dimensional arrangement of atoms

________Quantum Mechanics (the wave-particle duality of electrons)_______
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CBE4010 Introduction to Materials Science for Chemical Engineers
K. - K. Koo
2. ATOMIC STRUCTURE
Level of structures
Atomic structure: up to 10-10 m (0.1 nm ) or 1 A
Atomic arrangements: short range order 1-10 A(0.1-1nm)
Nanostructure: 1-100 nm ( ~10-9 to 10-7 m )
Microstructure: 0. 1-100 m ( ~ 10-7 to 10-4 m )
Macrostructure: 100 m 이상
2-1 Atomic Structure
Typical Atom:
Particle
Location
Mass
proton
nucleus
1.66x10-24 g (1 g/mole)
1.66x10
Charges
-24
neutron
nucleus
g (1 g/mole)
electron
outside nucleus 0.911x10-27 g (1/1823 g/mole)
+e
0
-e
e = electronic charge = 1.6 x10-19 coulomb
atomic weight - the weighted average of the atomic masses of the atom’s naturally
occurring isotopes.
atomic mass unit (amu)
Proton or neutron : 1.66x10-24 g =1amu
1amu/atom = 1g/mol
C12 : 12amu/atom, 12g/mol
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The Electronic Structure of the Atom
Electrons occupy discrete energy levels within the atom.
The energy level to which each electron belongs is determined by 4 quantum numbers:
The principal quantum number n (1, 2, 3, …)
The azimuthal quantum number l
The magnetic quantum number ml
The spin quantum number
ms
Rules for Filling Orbitals with Electrons
Rule 1. the Aufbau Principle
electrons are placed into orbitals in order of the energy of the orbital ; lowest
energy orbitals are filled first.
1s < 2s < 2p <
3s < 3p . . .
Rule 2. Hund's Rule
one electron is places in all orbitals of equal energy before two electrons are placed
in any one of these orbitals.
Rule 3. Pauli Exclusion principle
a maximum of two electrons can occupy an orbital.
No two electrons can have the same four quantum numbers.
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2-2 Atomic Bonding
What is a Chemical Bond?
- A chemical bond exists between two atoms or group of atoms when the force acting
between them lead to the formation of stable aggregate.
- When 2 atoms or more atoms are more stable as an aggregate, a chemical bond is formed.
What is the criteria for an unstable atom?- an incompletely filled quantum shell
How does the atom fill this cell?- by forming a chemical bond
Types of Bonding
(1) strong, primary or chemical bonds: Covalent Bond, Metallic Bond, Ionic Bond
(2) weak, secondary or Van der Waals bonds
- Keesom force: permanent dipole – permanent dipole
( including hydrogen bonds )
- Deye force: permanent dipole – induced dipole
- London or instantaneous ( or induced ) dipole bonds
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Metallic bond: by "sharing" of electrons over many atoms ; nondiretional
- delocalized over the entire aggregate
structure, forms energy band.
- strength of metallic bonds may be
gauged by the heat of sublimation of the
metal, ΔHsubl.
Metal (crystal) → Metal (isolated gas atom)
.
Metal
Mg
Al
Cu
Fe
Ti
ΔHsubl
148 kJ/mol
326
338
416
437
.
1.54 eV/atom
3.38
3.51
4.32
4.53
.
Covalent Bonding: by sharing of valence electrons; highly directional (definite bond
angle)
- molecular covalent : discrete molecular unit, H2O, CO2, HCl, . .
- network covalent : 3-D structure, no discrete unit, BeCl2, SiO2, .. .
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Ionic Bond : by electron transfer, nondirectional
ex) Na(g) + Cl (g)
i) Na(g)
ii) Cl(g) + e
iii) Na+(g)
→
→
→
NaCl (g)
Na+(g) + e
ionization energy of Na, ΔE1 = 5.14 eV
Cl - (g)
electron affinity of Cl , ΔE 2 = -3.65 eV
+ Cl- (g)
→
Na+ Cl- (g)
change in electrical PE, ΔE 3 = -5.70 eV
ΔE = ΔE 1 + ΔE 2 + ΔE 3 = -4.21 eV
Reaction is energetically favorable!!
- Formation of an Ionic Bond
Interatomic spacing: The equilibrium distance between atoms is caused by a balance
between repulsive and attractive forces.
FA = - k0 (Z1 q )(Z2 q ) / a2
q: the electronic charge (1.602 x 10-19 C)
Z: valence of charged ion ( +1 for Na+, -1 for Cl- )
FR = K / am or λe-a/ρ
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- Bonding Force Curve:
The equilibrium bond length: FA + FR = 0
F(a) = dE(a)/da
- Bonding Energy Curve:
E(a) = - A/a6 + B/a12 : LennardJones ( 6 - 12 ) potential
E(a) = - A/a6 + B exp( -k a ) :
( 6 - exp ) or
modified Buckingham potential
- Chemical bonds are neither purely covalent nor purely ionic.
electronegativity (≠ electron affinity ) difference ≥ 2
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→
ionic bond
Secondary, van der Waals or physical Bonds
- There are three types of van der Waals interactions:
1) London or Dispersion Forces: instantaneous dipole/induced dipole forces always
present between atoms, ions or molecules, but may be overshadowed by strong, primary
bonds.
2) Keesom interaction: interaction between molecules that are permanently polarized
molecules
- Polar molecules- covalent bonds between atoms of different electronegativities may be
polar.
- Dipole-Dipole Bonds: between molecules with permanent dipole monents or bond dipoles
- Hydrogen Bond: dipole-dipole force between molecules containing hydrogen bonded to a
very electronegative atom (O, F, Cl or N).
3) Debye interaction: Interaction between an induced dipole and a permanent dipole.
(example: water-carbon tetrachloride)
- van der Waals forces play a very important role in many areas of engineering.
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binding energies for the four bonding mechanisms
bond
ionic
covalent
metallic
hydrogen O-H…O
O-H…N
C-H…O
N-H…O
N-H…N
Intermolecular
binding energy
70-350 kcal/mol
50-200
27-83
6-7
4-7
2-3
2-3
~6
0.01-2
SUMMARY
Material
Metals
Bonding
Metallic
Examples
Fe, Cu
Ceramics
& Glasses
Ionic
Ionic/Covalent
Covalent
MgO, Al2O3
SiO2
SiC
Polymers
Covalent and
Van der Waals
PE, PVC
Electronic
Covalent
Covalent/Ionic
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Si
GaAs, ZnS