Transcript Atomic Structure

Material Properties
Atomic Structure determines:

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

Physical Properties
Chemical Properties
Biological Properties
Electromagnetic Properties
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Physical Properties
Density
Permeability
Mass
Moisture content
susceptibility
Structure
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Physical Properties
Specific gravity
Color
Texture
Shape
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Chemical Properties
Resistance to deterioration
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Oxidation
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Solubility
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Biological Properties
Bacterial growth
Hazard/exposure consideration
Resistance to infestation
Biodegradability
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Electromagnetic Properties
Conductance
Galvanic potential
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Material Selection
Strength
Serviceability
Deflections
Adaptability to future
Durability
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Material and Testing
Standards
ASTM /ASME Standards
AASHTO, State Highway,
EPA, HUD, US-Army
BOCA,ICBO, ICC, ISO
AISC/ACI/AITC
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minimum quality standards,
minimum application standards
Some may be performance standards
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Primary Bonds
Types of primary bond
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1. Ionic - transfer of electrons
 Metallic and non metallic elements
Na
Cl
 Sodium chloride salt
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2. Covalent - sharing with adjacent atoms
H
H
 Polymeric materials
 Hydrogen gas
Metal ions
Electron cloud
+
+
+
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3. Metallic - mass sharing of electrons
+
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+
+
+
+
 All metal
+
+
+
+
+
+
+
+
+
+
Ionic Bonds
Electrons transferred
Strong attractive
forces between atoms
Solids at room temps
High melting
temperature
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Ionic Bonds
Solution-good
conductors
Solid-poor
conductors
Soluble in polar
solvents, water
Insoluble in nonpolar
solvents, organic
solvents.
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Ionic Bonds
Low energy metals
bonding to high
energy nonmetals
Ca+2 +O-2 =
CaO
Exothermic in
formation
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Attractive Forces
NaCl
11
Force  c 
 c  0.131
2
.95  1.81
MgO
22
Force  c 
 c  0.951
2
.65  1.40
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Covalent Bonds
Electrons are shared
in joint orbital
Can lead to small
molecules with
polarity
No “bonding” between
molecules, but some
attraction and
repulsion (secondary
bonds).
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Covalent Bonds
Gases, liquids,
(mech. weak
substances)
Can lead to long
extended networks
Ceramics, diamond
(high binding energy)
Polymer chains (weak
between chains)
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Metallic Bonds
Metal – “element
with 1, 2, or 3,
valence electrons”
+ + +
+ + +
+
+ +
+
+
+ + +
+ + +
+
+ +
+ + +
+ + +
+ + +
+
+
+ +
+
+
+
No clearly defined
molecules
+
+
+
Equilibrium of
repulsive forces
Electron cloud &
Electronic bond
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Secondary Bonds - van der
Waals Bonds
Weak compared to primary bonds
Result from dipoles - electrostatic
attraction
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Dipole occurs when have separation of
positive and negative portion of atom or
molecule
Causes gasses to liquefy
Water also a dipole
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Cl
H
+
-
H
O
105°
H
Strengths of Different Types of
Bonds
Bonding Type
Material
Energy
kJ/mole
Ionic
NaCl
640
801
Covalent
Si
450
1410
Metallic
Fe
406
1538
Hydrogen
H2O
51
0
van der Waals
Cl
31
-101
Source: Callister, Materials Science and Engineering
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Melt
Temp.°C
Structure of Materials
Crystalline
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“Repeated pattern or arrangement of
atoms”
Ordered systems not necessarily
crystalline
 Laminar or small ordered systems
arranged in disorganized manner
Amorphous
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Random molecular structure
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Material Classification
Metallic Solids
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Metallic bonding
Steel, iron, aluminum, copper, other metals
Crystalline
Organic Solids
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Primarily covalent and van der Waals
bonding
Asphalt, plastics, wood
Largely amorphous (although not entirely)
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*Note the change
Material Classification
Inorganic Solids (ceramics)
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Primarily ionic and covalent bonding
Portland cement, bricks, glass,
aggregates, minerals
Largely crystalline (but not entirely)
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Bonding and Structure of
Materials (Generally Speaking)
Material
Bonding
Structure
Steel
Metallic
Crystalline
Aggregates
/ Minerals
Ionic, Covalent
Crystalline, Some
Amorphous
Portland
Cement
Ionic, Covalent, van der
Waals
Amorphous,
Crystalline
Asphalt
Covalent, van der Waals Amorphous
Polymers
Covalent, van der Waals Amorphous
Wood
Covalent, van der Waals Crystalline,
Amorphous
Glass
Covalent
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Amorphous
(unaltered)
Crystalline Structures
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Crystalline Materials
Atoms arranged in repeating and regular array
Unit cells  individual crystals structural
part
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Unit cell - smallest repeating unit
Body centered cubic (BCC)
Face centered cubic (FCC)
Hexagonal close-packed (HCP)
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Concepts of Crystalline
Structure
Coordination number
Number of “nearest neighbors”
Here 8 for BCC
One at each corner
Atomic Packing Factor (APF)
APF = Volume of atoms in cell
Total volume of cell
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Body Center Cubic Structure
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Face Centered Cubic
Structure
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Hexagonally Close Packed
6 around 1 on top
6 around 1 on
bottom
3 at mid-height
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Body Center Cubic Structure
Pure Iron
(600°C to 910°C)
Low Carbon Steel
(723°C to ~1400°C)
Some Aluminum Alloys
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Face Centered Cubic
Structure
Pure aluminum
(-269°C to melting)
Pure iron
(910°C to 1403°C )
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Defects in Crystals
Point
Line
Area
Volume
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Point Defects
Crystal contains many - many unit cells
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Explain permanent (plastic) deformation in metals
Defects
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Interstitial
 vacancy - missing
 interstitial - extra
Impurities
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Interstitial - extra
 Carbon in iron
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Substitutional
 Copper alloy in nickel
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Source: Callister, Materials Science and Engineering
Lattice Defects
Imperfections in
arrangements of atoms
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edge dislocation - line defect
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Arrangements of Crystals
Grain boundary
Types of Interfaces?
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Amorphous Structure
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