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READING Chapter 1 in DeGraef and McHenry, All;

ap er n e rae an c enry, pp. - . Chapter on bonding from your Introduction to Materials book.

REFERENCE: Chapter 1 in Rohrer

Prof. M.L. Weaver

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What do we mean by structure?What do we mean by structure?What do we mean by structure?What do we mean by structure? We are concerned with how to quantitatively describe

arranged at relevant length scales.

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Structural Length Scales1 m1098

Radio

Electrons Neutrons Photonsnm

Can seen it w/ unaided eye.

1 mm

107

106Microwaves

totelescopes

methods)

Macro

Need a microscope to see it.

105

104Infrared

Microscopes

tronscatte

rin

EM,

SEM

)

Micro

1 m103

102UV

ge(v

ariousne

icationrange(

no

Visible

light

.

1 nm

101

100

rays

hods

pplicationra

Appl

ic

Na

size of an atom (~1 = 0.1 nm).10

-1

10-2

ays

X

Diffractionme

nAto

Prof. M.L. Weaver

FigureadaptedfromM.DeGraefandM.E.McHenry,StructureofMaterials,CambridgeUniversityPress,CambridgeUK(2007)p.5.

1 pm10-

10-4Gamma

Nuclear

Sub-atomic.

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What do we mean by structure?What do we mean by structure?What do we mean by structure?What do we mean by structure? Atomic structure. Atoms in a material are bound to

Metallic bonds

Covalent bonds

Secondar bonds

Atomic arran ements also rovide useful tructuraldescriptors.Crystalline structure

Prof. M.L. Weaver

Amorphous structure

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What do we mean by structure?What do we mean by structure?What do we mean by structure?What do we mean by structure? Microstructure: Arrangements of large groups of

.Phase/Crystal Morphology

Phase/Crystal Distribution

EtcStructural Feature Dimension(m)

10

Missing/extraatoms ~1010

Crystals(orderedatoms) 108 101

Prof. M.L. Weaver

Secondphase particles 108 104

Crystaltexturing >106

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Crystal Structure DescriptorsCrystal Structure DescriptorsCrystal Structure DescriptorsCrystal Structure Descriptors Forcrystalline materials, descriptors are based on the

theory ofcrystallography.

NaCl

rock salt

All atoms have the same average environment around

(green Cl- ; yellow Na+)

calcite

Prof. M.L. Weaver

them. This constitutes long-range order (LRO) which is

a major structure descriptor for crystals.

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NonNon--crystalline Structuralcrystalline StructuralNonNon--crystalline Structuralcrystalline Structural

If a material isnon-crystalline, it lacks spatial. ., .

Silica

glass

On a smaller length scale, the atoms may exhibit

http://www.pisces-press.com/C-Nav/images/silica%20glass-b.jpghttp://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Silica.svg/2000px-Silica.svg.png

Prof. M.L. Weaver

short-range order (SRO), which is a major

descriptor for the structure of glassy materials.

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Example of LRO and SROExample of LRO and SRO

From Prof. T. Scharf

University of North Texas

LRO SRO

-(amorphous) on singlecrystal Si wafer

Prof. M.L. Weaver

MFET 3450 14

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Materials are not perfectlyMaterials are not perfectly

ordered arra s of atom orordered arra s of atom or ionsionsPolycrystalline ZnO thin film

O2-

Zn2+

Planar stacking faults (2-D

area defect) denoted by

arrows

- An interfacial defect that exists withinabove hexagonal closed packed (HCP)stacking sequencedisruption in lattice.

Prof. M.L. Weaver

From Prof. T. Scharf

University of North Texas

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Materials are not perfectlyMaterials are not perfectly

ordered arra s of atom orordered arra s of atom or ionsionsPolycrystalline ZnO thin film

Zoom in on box from previous page:

. .

University of North Texas

-

line defect) denoted by

arrow

Prof. M.L. Weaver

16

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BondingBondingBondingBonding

The physical and chemical properties of materials arestrongly influenced by interatomic bonds. Thus they aren uence y e ou ermos va ence e ec rons.

Can we determine a materials structure from knowled eof chemical bonding?

NO!

Many examples in terms of structural state and bonding, e.g.,

TiCl2 (s) and TiCl4 (l)

Must be careful assigning particular types of bonds to agiven collection of atoms/ions.

Prof. M.L. Weaver

Cant just use stoichiometric formulas to predict material

structure.

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PropertiesPropertiesPropertiesProperties Refer to how a material responds to an external

. Isotropic non-directional

Tem erature

Gravity/mass

Etc n sotrop c rect ona

Stress

Elastic modulus

Electrical conductivity Etc

Prof. M.L. Weaver

rys a ograp y p ays a ro e. ymme ry.

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Symmetry ExampleSymmetry ExampleSymmetry ExampleSymmetry Example Consider the 2-D material illustrated to the right.

. .

Apply an electric field (E) alongx-axis. Generate acertain current density (j). x

y

Apply an electric field alongy-axis. Should generate the same

current density because structure is same inx

andy

directions. .

x xx xy xj E

E

Properties reflect

x xx xy xj E

E

structure. In this casethey are the same inxandy.

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Prof. M.L. Weaver