7/3/2012

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Modern

Materials

© 2009, Prentice-Hall, Inc.

Chapter 12

Modern Materials

John D. Bookstaver

St. Charles Community College

Cottleville, MO

Chemistry, The Central Science, 11th edition

Theodore L. Brown, H. Eugene LeMay, Jr.,

and Bruce E. Bursten

Modern

Materials

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Chapter 12 Problems

• Problems 1, 4, 7, 9, 11, 13, 23, 29,

3143, 53, 55

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Materials

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Modern Materials

• Conductors, Semiconductors, and

Insulators

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Materials

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Delocalized Bonding in Sodium Metal

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Materials

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Bonding in Metals Band theory.

• Extension of MO

theory.

N atoms give N orbitals

that are closely spaced

in energy.

• N/2 are filled.

The valence band.

• N/2 are empty.

The conduction band.

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Materials

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Band Theory

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Types of Materials

Recall that atomic

orbitals mix to give

rise to molecular

orbitals.

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Materials

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Types of Materials

In such compounds,

the energy gap

between molecular

orbitals essentially

disappears, and

continuous bands of

energy states result.

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Materials

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Types of Materials

Rather than having molecular

orbitals separated by an energy

gap, these substances have energy

bands.

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Materials

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Types of Materials

The gap between bands determines

whether a substance is a metal, a

semiconductor, or an insulator.

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Materials

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Types of Materials

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Materials

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Metals

Valence electrons

are in a partially-

filled band.

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Materials

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Metals

• There is virtually no

energy needed for

an electron to go

from the lower,

occupied part of the

band to the higher,

unoccupied part.

• This is how a metal

conducts electricity.

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Materials

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Semiconductors

Semiconductors

have a gap between

the valence band

and conduction

band of ~50-300

kJ/mol.

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Materials

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Semiconductors

• Among elements, only silicon,

germanium and graphite

(carbon), all of which have 4

valence electrons, are

semiconductors.

• Inorganic semiconductors (like

GaAs) tend to have an

average of 4 valence electrons

(3 for Ga, 5 for As).

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Modern

Materials

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Doping

By introducing very

small amounts of

impurities that have

more (n-Type) or fewer

(p-Type) valence

electrons, one can

increase the

conductivity of a

semiconductor.

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Materials

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Insulators

• The energy band

gap in insulating

materials is

generally greater

than ~350 kJ/mol.

• They are not

conductive.

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Materials

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Modern Materials

• Ceramics

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Modern

Materials

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Ceramics

• These are inorganic solids, usually hard and brittle.

• They are highly resistant to heat, corrosion and wear.

– Ceramics do not deform under stress.

– They are much less dense than metals, and so are used in

place of metals in many high-temperature applications.

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Materials

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Superconductors

At very low

temperatures, some

substances lose

virtually all

resistance to the

flow of electrons.

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Materials

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Superconductors

Much research has

been done recently

into the development

of high-temperature

superconductors.

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Materials

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Superconductors

The development of

higher and higher

temperature

superconductors will have

a tremendous impact on

modern culture.

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Materials

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Polymers

Polymers are molecules of high molecular

mass made by sequentially bonding

repeating units called monomers.

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Some Common Polymers

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Materials

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Addition Polymers

Addition polymers are made by coupling the

monomers by converting -bonds within

each monomer to -bonds between

monomers.

Ethylene Polyethylene

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Materials

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Condensation Polymers

• Condensation polymers are made by joining

two subunits through a reaction in which a

smaller molecule (often water) is also formed

as a by-product.

• These are also called copolymers.

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Materials

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Synthesis of Nylon

Nylon is one

example of a

condensation

polymer.

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Materials

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Properties of Polymers

Interactions

between chains of a

polymer lend

elements of order to

the structure of

polymers.

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Materials

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Properties of Polymers

Stretching the polymer chains as they form

can increase the amount of order, leading to

a degree of crystallinity of the polymer.

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Materials

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Properties of Polymers

Such differences in

crystallinity can lead

to polymers of the

same substance

that have very

different physical

properties.

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Modern

Materials

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Cross-Linking

Chemically bonding

chains of polymers

to each other can

stiffen and

strengthen the

substance.

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Materials

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Cross-Linking

Naturally-occurring rubber is too soft and

pliable for many applications.

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Materials

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Cross-Linking

In vulcanization, chains are cross-linked by

short chains of sulfur atoms, making the

rubber stronger and less susceptible to

degradation.

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Modern

Materials

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Ceramics

Ceramics are made

from a suspension

of metal hydroxides

(called a sol).

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Materials

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Ceramics

These can undergo

condensation to

form a gelatinous

solid (gel), that is

heated to form a

metal oxide, like the

SiO2 shown here.

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Materials

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Biomaterials

• Materials used in the

body must

– be biocompatible,

– have certain physical

requirements, and

– have certain chemical

requirements.

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Materials

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Biomaterials

• Biocompatibility

– The materials used

cannot cause

inflammatory

responses.

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Materials

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Biomaterials

• Physical Requirements

– The properties of the

material must mimic the

properties of the “real”

body part (i.e.,

flexibility, hardness,

etc.).

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Materials

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Biomaterials

• Chemical Requirements

– It cannot contain even

small amounts of

hazardous impurities.

– Also it must not degrade

into harmful substances

over a long period of

time in the body.

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Modern

Materials

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Biomaterials

• These substances

are used to make:

– Heart valves

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Materials

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Biomaterials

• These substances

are used to make:

– Heart valves

– Vascular grafts

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Materials

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Biomaterials

• These substances

are used to make:

– Heart valves

– Vascular grafts

– Artificial skin grafts

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Modern

Materials

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Electronics

• Silicon is very

abundant, and is a

natural

semiconductor.

• This makes it a

perfect substrate for

transistors,

integrated circuits,

and chips.

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Materials

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Electronics

In 2000, Alan J. Heeger, Alan G. MacDiarmid, and

Hideki Shirakawa won a Nobel Prize for the

discovery of “organic semiconductors” like the

polyacetylene below.

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

H H H H H H H H H

HHHHHHHHH

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Materials

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Electronics

Noncrystalline silicon

panels can convert

visible light into

electrical energy.

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Modern

Materials

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Liquid Crystals • Some substances do

not go directly from

the solid state to the

liquid state.

• In this intermediate

state, liquid crystals

have some traits of

solids and some of

liquids.

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Materials

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Liquid Crystals

Unlike liquids, molecules in liquid crystals

have some degree of order.

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Liquid Crystals

In nematic liquid crystals, molecules are only

ordered in one dimension, along the long

axis.

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Modern

Materials

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Liquid Crystals

In smectic liquid crystals, molecules are

ordered in two dimensions, along the long

axis and in layers.

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Liquid Crystals

In cholesteryl liquid

crystals, nematic-

like crystals are

layered at angles to

each other.

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Materials

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Liquid Crystals

These crystals can

exhibit color

changes with

changes in

temperature.

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Modern

Materials

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Light-Emitting Diodes

In another type of

semiconductor, light

can be caused to be

emitted (LEDs).

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Materials

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Nanoparticles

Different sized

particles of a

semiconductor (like

Cd3P2) can emit

different wavelengths

of light depending on

the size of the energy

gap between bands.

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Materials

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Nanoparticles

Finely divided metals

can have quite different

properties than larger

samples of metals.

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Modern

Materials

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Carbon Nanotubes

Carbon nanotubes

can be made with

metallic or

semiconducting

properties without

doping.