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NCSU
The World of Atoms
Instructor: Dr. Gerd Duscher http://www4.ncsu.edu/~gjdusche
email: [email protected]
Office: 2156 Burlington Nuclear Lab.Office Hours: Tuesday: 10-12pm
Objective today: How do atoms arrange themselves ? Why is symmetry important ?
Why do atoms break symmetry?
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What is an Atoms?
Bohr Model
that is too simple
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Ionic Bonding
+ -
Covalent Bondingshared electrons from carbon atom
shared electrons from hydrogen atoms
H
H
H
H
C
CH4
arises from interaction between dipoles-ex: liquid HClasymmetric electron
clouds
+ - + -van der Waals
bonding
H Cl H Clvan der Waals
bonding
Van Der Waals Bonding
How do they bond?
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• bond length, r
• bond energy, Eo
• melting temperature, Tm
Tm is larger if Eo is larger.
What properties does that imply?
F F
r
r
larger T m
smaller T m
Energy (r)
r o
Eo=
“bond energy”
Energy (r)
r o r
unstretched length
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Ceramics(Ionic & covalent bonding):
Metals(Metallic bonding):
Polymers(Covalent & Secondary):
large bond energylarge Tm
large Esmall
variable bond energymoderate Tm
moderate Emoderate
directional Propertiesvan der Waals bonding dominates
small Tsmall Elarge
Summary: Primary Bonds
secondary bonding
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• Non dense, random packing
• Dense, regular packing
Dense, regular-packed structures tend to have lower energy.
Energy And Packing
r
typical neighbor bond length
typical neighbor bond energy
ener
gy
r
typical neighbor bond length
typical neighbor bond energy
ener
gy
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• atoms pack in periodic, 3D arrays• typical of:
Crystalline materials...
-metals-many ceramics-some polymers
• atoms have no periodic packing• occurs for:
Noncrystalline materials...
-complex structures-rapid cooling
Si Oxygen
crystalline SiO2
noncrystalline SiO2"Amorphous" = Noncrystalline
Materials And Packing
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• tend to be densely packed.
• have several reasons for dense packing:-Typically, only one element is present, so all atomic radii are the same.-Metallic bonding is not directional.-Nearest neighbor distances tend to be small in order to lower bond energy.
• have the simplest crystal structures.
We will look at three such structures...
Metallic Crystals
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• rare due to poor packing (only Po has this structure)• close-packed directions are cube edges.
• Coordination # = 6 (# nearest neighbors)
Simple Cubic Structure (sc)
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• Coordination # = 8
• Close packed directions are cube diagonals.--Note: All atoms are identical; the center atom is shaded differently only for ease of viewing.
Body Centered Cubic Structure (bcc)
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• Coordination # = 12
• Close packed directions are face diagonals.--Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.
Face Centered Cubic Structure (fcc)
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• ABCABC... stacking sequence• 2D projection
• fcc unit cellA
BC
fcc Stacking Sequence
A sites
B sites
C sitesB B
B
BB
B BC C
CA
A
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12
• Coordination # = 12
• ABAB... Stacking Sequence
• APF = 0.74
• 3D Projection • 2D Projection
A sites
B sites
A sites Bottom layer
Middle layer
Top layer
Hexagonal Close-Packed Structure (hcp)
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graphite
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Diamond Structure
silicon, diamond ZnS – type (GaAs)
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• Compounds: Often have similar close-packed structures.
• Close-packed directions --along cube edges.
• Structure of NaCl
Structure Of Compounds: Nacl
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Perovskite Strucutre
SrTiO3
Applications: non-linear resistors (PTC), SMD capacitors, piezoelectric sensors and actuators, ferroelectric memory.
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Why? Metals have... • close-packing (metallic bonding) • large atomic mass Ceramics have... • less dense packing (covalent bonding) • often lighter elements Polymers have... • poor packing (often amorphous) • lighter elements (C,H,O) Composites have... • intermediate values
Densities Of Material Classes
metals ceramics polymers
(g
/cm
3)
Graphite/ Ceramics/ Semicond
Metals/ Alloys
Composites/ fibersPolymers
1
2
20
30Based on data in Table B1, Callister *GFRE, CFRE, & AFRE are Glass,
Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on
60% volume fraction of aligned fibers in an epoxy matrix). 10
345
0.30.40.5
Magnesium
Aluminum
Steels
Titanium
Cu,Ni
Tin, Zinc
Silver, Mo
Tantalum Gold, W Platinum
Graphite Silicon
Glass-soda Concrete
Si nitride Diamond Al oxide
Zirconia
HDPE, PS PP, LDPE
PC
PTFE
PET PVC Silicone
Wood
AFRE*
CFRE*
GFRE*
Glass fibers
Carbon fibers
Aramid fibers
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• Some engineering applications require single crystals:
• Crystal properties reveal features of atomic structure.
--Ex: Certain crystal planes in quartz fracture more easily than others.
--diamond single crystals for abrasives
--turbine blades
Crystals as Building Blocks
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• Most engineering materials are polycrystals.
• Nb-Hf-W plate with an electron beam weld.• Each "grain" is a single crystal.• If crystals are randomly oriented, overall component properties are not directional.• Crystal sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers).
1 mm
POLYCRYSTALS
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• Single Crystals-properties vary with direction: anisotropic.
-example: the modulus of elasticity (E) in bcc iron:
• Polycrystals
-properties may/may not vary with direction.-if grains are randomly oriented: isotropic. (Epoly iron = 210 GPa)-if grains are textured, anisotropic.
200 mm
Single vs Polycrystals
E (diagonal) = 273 GPa
E (edge) = 125 GPa
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TEMs at NCSUThe NEW JEOL 2010F
This is a TEM/STEM, which can do everything
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TEMs at NCSUTEM Lab Course at the OLD TEM: Topcon
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STEM at ORNL
This STEM provides the smallest beam in the world.
It uses the brightest sourcein the universe,1000 times brighter thana supernova.
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Why?
• before deformation • after tensile elongation
slip steps
That is what happens when pulling wires.
Dislocation move, more dislocation get generated and entangle (interact) with themselfs, and other defects.
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• Dislocations slip planes incrementally...• The dislocation line (the moving red dot)... ...separates slipped material on the left from unslipped material on the right.
Simulation of dislocationmotion from left to rightas a crystal is sheared.
Incremental Slip
push
fixed
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• Dislocation motion requires the successive bumping of a half plane of atoms (from left to right here).• Bonds across the slipping planes are broken and remade in succession.
Atomic view of edgedislocation motion fromleft to right as a crystalis sheared.
Bond Breaking And Remaking
push
fixed
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• Vacancies:-vacant atomic sites in a structure.
Vacancydistortion of planes
• Self-Interstitials:-"extra" atoms positioned between atomic sites.
self-interstitialdistortion
of planes
Point Defects
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Vacancy in Silicon
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Two outcomes if impurity (B) added to host (A):• Solid solution of B in A (i.e., random dist. of point defects)
• Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)
OR
Substitutional alloy(e.g., Cu in Ni)
Interstitial alloy(e.g., C in Fe)
Second phase particle--different composition--often different structure.
Point Defects In Alloys
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Imaging of Single Bi Atoms in Si(110)
A. Lupini, VG HB501UX with Nion Aberration Corrector, 100 kV
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• Vacancy atoms• Interstitial atoms• Substitutional atoms• Anti-site defects
• Dislocations
• Grain Boundaries
Point defects(0 dimensinal)
Line defects(1 dimensional)
Area defects(2dimensional)
Types of Imperfections