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STRUCTURE AND PROPERTIES

OF MATERIALS

The Structure of Crystalline Solids

Lecture _3_Cont.

September 24, 2010 ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

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CRYSTALLOGRAPHICCrystallography is the experimental science of determining the

arrangement of atoms in solids. In older usage, it is the scientific

study of crystals.

The word "crystallography" is derived from the Greek words

crystallon = cold drop / frozen drop, with its meaning extending

to all solids with some degree of transparency, and graphein =

write.

Crystallography is a tool that is often employed by materials

scientists. In single crystals, the effects of the crystalline

arrangement of atoms is often easy to see macroscopically,

because the natural shapes of crystals reflect the atomic structure.

In addition, physical properties are often controlled by crystalline

defects. The understanding of crystal structures is an important

prerequisite for understanding crystallographic defects.

September 24, 2010

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Structure and Properties of Materials

Nahid Abdel Salam

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CRYSTALLOGRAPHIC

Representation of:

Points

Directions, and In cubic systems

Planes

Calculate linear and planar densities

In Tutorial Session from page 64 to 76

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3ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

SIGNIFICANCE OF CRYSTALLOGRAPHIC

DIRECTIONS AND PLANES

Crystallographic directions are used to indicate aparticular orientation of a single crystal or of an orientedpolycrystalline material;

Metals deform more easily, in directions along which atomsare in closest contact;

It is much easier to magnetize iron in the [100] directioncompared to [111] or [110];

Metals deform along planes of atoms that are most tightlypacked together;

In electronic materials, we need to be sure the substrate isoriented in such way that the thin film can grow on aparticular crystallographic plane.

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

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SINGLE CRYSTAL

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5ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

A homogenous solid

formed by a repeating,

three-dimensional

pattern of atoms, ions, or

molecules and having

fixed distances between

constituent parts

Single crystal of silicon

USES OF SINGLE CRYSTAL

Monocrystals of silicon and other semiconductors are

important for manufacture of integrated circuit.

Monocrystals of sapphire and other materials are used

for lasers and nonlinear optics.

Monocrystals of fluorite are sometimes used in the

objective lenses of apochromatic refracting telescopes

Monocrystals of metals, especially superalloys, are used

for their special mechanical properties. Turbine blades of

some gas turbines are made of single crystal cast

superalloy.

Monocrystals of copper (crystalline copper) are used for

fine crystalline powders and hi tech.

September 24, 2010

6ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

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SINGLE-CRYSTAL SEMICONDUCTOR WIRE

BUILT INTO AN OPTICAL FIBER

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

ScienceDaily (Mar. 17, 2008) — An

international science team from Penn State

University in the United States and the

University of Southampton in the United

Kingdom has developed a process for growing

a single-crystal semiconductor inside the

tunnel of a hollow optical fiber. The device

adds new electronic capabilities to optical

fibers, whose performance in electronic devices

such as computers typically is degraded by the

interface between the fiber and the device. The

research is important because optical fibers --

which are used in a wide range of technologies

that employ light, including

telecommunications, medicine, computing, and

remote-sensing devices -- are ideal media for

transmitting many types of signals.

Single-crystal semiconductor

wires integrated into

microstructured optical wires.

POLYCRYSTALLINE STRUCTURE

September 24, 2010

8ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

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AMORPHOUS STRUCTURE

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

Amorphous solid substance does

not possess long-range order of

atoms positions. Some liquids when

cooled become more and more

viscous and then rigid, retaining

random atom characteristic

distribution.

This state is called under cooled

liquid or amorphous solid.

Common glass, most of Polymers,

glues and some of Ceramics are

amorphous solids. Some of the

Metals may be prepared in

amorphous solid form by rapid

cooling from molten state.

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

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SINGLE VS. POLY CRYSTALS

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

Single crystal

Properties vary with direction Anisotropic material

Example of Anisotropy: BCC iron

Anisotropy (not

isotropic) is the

property of being

directionally

dependent.

SINGLE VS. POLY CRYSTALS

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

Poly-crystals

Properties may/may not vary with direction

Example: Common steel is polycrystalline

If grains are random Isotropic E ≈ 210 GPa

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

Definitions

Study well the following:

Polymorphism page 61 Example

Allotropy page 61 Iron -Tin

Isotropy and Anisotropy page 82

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

Polymorphism is a physical phenomenon where

a material may have more than one crystal

structure. A material that shows polymorphism

exists in more than one type of space lattice in

the solid state. If the change in structure is

reversible, then the polymorphic change is

known as allotropy.

Polymorphism and Allotropy

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

One familiar example is found in

carbon: graphite is the stable

polymorph at ambient conditions,

whereas diamond is formed at

extremely high pressures.

Example (Polymorphism)

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

Diamond and

graphite are two

allotropes of carbon:

pure forms of the

same element that

differ in structure.

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

a) Diamond,

b) Graphite,

c) Lonsdaleite,

d) C60

(Buckminsterfullere

ne or buckyball),

e) C540,

f) C70,

g) Amorphous carbon,

h) single-walled

carbon nanotube or

buckytube.

Eight allotropes of carbon:

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ENGR 230

Structure and Properties of Materials

Nahid Abdel Salam

The best known example for allotropy

is iron. When iron crystallizes at

1538°C it is B.C.C. (δ-iron), at 1394°C

the structure changes to F.C.C. (γ-iron

or austenite), and at 912°C it again

becomes B.C.C. (α-iron or ferrite).

Example (Allotropy)