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The World of Atoms

Instructor: Dr. Gerd Duscher http://www4.ncsu.edu/~gjdusche

email: gerd_duscher@ncsu.edu

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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• 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)

NCSUHexagonal Close-Packed Structure (hcp)

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

1500

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1500

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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