NCSUNCSU

[110]

[001]

[110]

SiSi

GaAsGaAs

2 nm2 nm

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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

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The World of Atoms

ObjectiveJohann Wolfgang von Goethe (1749-1832): Faust

Faust is searching for the first principles, for "that inner force which holds the world together," “was die Welt im Innersten zusammenhält”.

So do we, in this lecture

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Atomic Structure

Bohr Model

that is too simple

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Atomic Structure

pro

bab

ilit

y1

Bohr Model

Nucleus

distance

Wavemechanic Model

Nucleus

electrons

pro

bab

ilit

y

1

radial distanceEnergy

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Some Links

Bonding :http://info.lu.farmingdale.edu/depts/met/met205/atomicbonds.html (pictures of different kinds of bonding)

Orbitals : http://web.mit.edu/3.091/www/orbs/ (computer simulation of electron positiond for certain orbitals)

Crystal Structure : http://web.mit.edu/3.091/www/cryst/(Pictures of several crystal structures)

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

3

Electrons...

Adapted from Fig. 2.5, Callister 6e.

Electron Energy States

incr

easi

ng

en

erg

y

n=1

n=2

n=3

n=4

1s2s

3s2p

3p

4s4p

3d

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• have complete s and p subshells• tend to be unreactive.

Adapted from Table 2.2, Callister 6e.

Stable Energy Configurations

Z Element Configuration

2 He 1s2

10 Ne 1s22s22p6

18 Ar 1s22s22p63s23p6

36 Kr 1s22s22p63s23p63d104s24p6

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• Why? Valence (outer) shell usually not filled completely.

• Most elements: Electron configuration not stable.

Adapted from Table 2.2, Callister 6e.

Survey of Elements

Element Hydrogen Helium Lithium Beryllium Boron Carbon ... Neon Sodium Magnesium Aluminum ... Argon ... Krypton

Electron configuration 1s 1

1s 2 (stable) 1s 22s 1 1s 22s 2 1s 22s 22p 1 1s 22s 22p 2 ...

1s 22s 22p 6 (stable) 1s 22s 22p 63s 1 1s 22s 22p 63s 2 1s 22s 22p 63s 23p 1 ...

1s 22s 22p 63s 23p 6 (stable) ...

1s 22s 22p 63s 23p 63d 10 4s 24p 6 (stable)

Atomic # 1 2 3 4 5 6

10 11 12 13

18 ... 36

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• Columns: Similar Valence Structure

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

Adapted from Fig. 2.6, Callister 6e.

The Periodic Table

He

Ne

Ar

Kr

Xe

Rn

iner

t gas

es

acce

pt 1

e-

acce

pt 2

e-

give

up

1e-

give

up

2e-

gi

ve u

p 3e

-

F Li Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

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Bondingattractive force FA

repulsive force FR

Net force FNre

pu

lsio

nat

trac

tion

0fo

rce F

repulsive energy ER

attractive energy EA

net energy EN

rep

uls

ion

attr

acti

on

0

pot

enti

al e

ner

gy E

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• bond length, r

• bond energy, Eo

• melting temperature, Tm

Tm is larger if Eo is larger.

Properties from Bonding: Tm

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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16

• Elastic modulus, E

• E ~ curvature at ro

Properties from Bonding: E

cross sectional area A o

DL

length, Lo

F

undeformed

deformed

r

larger Elastic Modulus

smaller Elastic Modulus

ener

gy

r o unstretched length

E is larger if E0 is more negative.

• coefficient of thermal expansion,

• ~ symmetry at ro

smaller

larger

r o

DL

length, Lo

unheated, T 1

heated, T 2

ren

ergy

is smaller if Eo is more negative

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• Occurs between + and - ions.

• Requires electron transfer.

• Large difference in electronegativity required.

• Example: NaCl

Ionic Bonding

Na (metal) unstable

Cl (nonmetal) unstable

electron

+ - Coulomb attraction

Na (cation) stable

Cl (anion) stable

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Ionic Bonding

+ -

Electron density difference

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• predominant bonding in ceramics

give up electrons acquire electrons

Examples: Ionic Bondings

He -

Ne -

Ar -

Kr -

Xe -

Rn -

F 4.0

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Ca 1.0

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

Cs Cl

MgO

CaF 2

NaCl

O 3.5

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• requires shared electrons

• example: CH4

C: has 4 valence e, needs 4 more

H: has 1 valence e, needs 1 more

Electronegativities are comparable.

Covalent Bonding

shared electrons from carbon atom

shared electrons from hydrogen atoms

H

H

H

H

C

CH4

H

H

H

H

Cenhanced electrondensity

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Ni3Al–Superalloy Bonds Covalently

Ni

Al

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• Molecules with nonmetals• Molecules with metals and nonmetals• Elemental solids (RHS of Periodic Table)• Compound solids (about column IVA)

Examples: Covalent Bonding

He -

Ne -

Ar -

Kr -

Xe -

Rn -

F 4.0

Cl 3.0

Br 2.8

I 2.5

At 2.2

Li 1.0

Na 0.9

K 0.8

Rb 0.8

Cs 0.7

Fr 0.7

H 2.1

Be 1.5

Mg 1.2

Ca 1.0

Sr 1.0

Ba 0.9

Ra 0.9

Ti 1.5

Cr 1.6

Fe 1.8

Ni 1.8

Zn 1.8

As 2.0

Si C

C(diamond)

H2O

C 2.5

H2

Cl2

F2

Si 1.8

Ga 1.6

GaAs

Ge 1.8

O 2.0

colu

mn

IVA

Sn 1.8Pb 1.8

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• Arises from a sea of donated valence electrons (1, 2, or 3 from each atom).

• Primary bond for metals and their alloys

Metallic Bonding

+ + +

+ + +

+ + +

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arises from interaction between dipoles

• permanent dipoles-molecule induced

• fluctuating dipoles

-general case:

-ex: liquid HCl

-ex: polymer

Van Der Waals Bonding

secondary bonding

HH HH

H2 H2

van der Waals bonding

ex: liquid H2asymmetric electron clouds

+ - + -van der Waals

bonding

+ -van der Waals

bonding + -

H Cl H Clvan der Waals

bonding

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Van Der Waals Bonding

permanent dipolsfluctuating (Induced) dipols

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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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Perovskite Strucutre

SrTiO3

Applications: non-linear resistors (PTC), SMD capacitors, piezoelectric sensors and actuators, ferroelectric memory.

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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.