Chapter 12 Modern Materials

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Nov 15, 2013 (3 years and 9 months ago)

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Modern

Materials

Chapter 12

Modern Materials

John D. Bookstaver

St. Charles Community College

St. Peters, MO



2006, Prentice Hall, Inc.

Chemistry, The Central Science
, 10th edition

Theodore L. Brown; H. Eugene LeMay, Jr.;
and Bruce E. Bursten

Modern

Materials

Types of Materials


Recall that atomic
orbitals mix to give
rise to molecular
orbitals.

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Materials

Types of Materials


As the number of
atoms grows, so
does the number of
molecular orbitals.

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Materials

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

Types of Materials


Rather than molecular orbitals
separated by an energy gap,
these substances have energy
bands.

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Materials

Types of Materials


The gap between bands
determines whether a
substance is a
metal
, a
semiconductor
, or an
insulator
.

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Materials

Types of Materials

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Materials

Metals


Valence electrons
are in a partially
filled band.

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Materials

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

Semiconductors


Semiconductors have a
gap between the valence
band and conduction
band of ~50 to 300 J/mol

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Materials

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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Materials

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

Insulators


The energy band
gap in insulating
materials is
generally greater
than ~350 kJ/mol.


They are not
conductive.

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Materials

Ceramics


They are inorganic solids, usually hard and brittle.


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 their place in many high
-
temperature
applications.

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Materials

Superconductors


At very low
temperatures, some
substances lose
virtually all
resistance to the
flow of electrons.

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Materials

Superconductors


Much research has
been done recently
into the development
of high
-
temperature
superconductors.

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Materials

Superconductors


The development of
higher and higher
temperature
superconductors will have
a tremendous impact on
modern culture.

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Materials

Polymers


Molecules of high molecular mass made by
sequentially bonding repeating units called
monomers.

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Materials

Some Common Polymers

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Materials

Addition Polymers


Made by coupling the monomers by
converting

-
bonds within each monomer to

-
bonds between monomers.

Ethylene

Polyethylene

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Materials

Condensation Polymers:


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

Synthesis of Nylon


Nylon is one
example of a
condensation
polymer.

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Materials

Properties of Polymers


Interactions
between chains of a
polymer lend
elements of order to
the structure of
polymers.

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Materials

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

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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Materials

Cross
-
Linking


Chemically bonding
chains of polymers
to each other can
stiffen and
strengthen the
substance.

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Materials

Cross
-
Linking


Naturally occurring rubber is too soft and
pliable for many applications.

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Materials

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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Materials

Ceramics


Made from a
suspension of metal
hydroxides (called a
sol
)

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Materials

Ceramics


These can undergo
condensation to
form a gelatinous
solid (
gel
), that is
heated to form a
metal oxide, like the
SiO
2
shown here.

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Materials

Biomaterials


Materials must


Be biocompatible.


Have certain physical
requirements.


Have certain chemical
requirements.

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Materials

Biomaterials


Biocompatibility


Materials cannot cause
inflammatory
responses.

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Materials

Biomaterials


Physical
Requirements


Properties must mimic
the properties of the
“real” body part (e.g.,
flexibility, hardness,
etc.).

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Materials

Biomaterials


Chemical
Requirements


Cannot contain even
small amounts of
hazardous impurities.


Cannot degrade into
harmful substances
over a long period of
time in the body.

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Materials

Biomaterials


These substances
are used to make:


Heart valves

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Materials

Biomaterials


These substances
are used to make:


Heart valves


Vascular grafts

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Materials

Biomaterials


These substances
are used to make:


Heart valves


Vascular grafts


Artificial skin grafts

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Materials

Biomaterials


These substances
are used to make:


Heart valves


Vascular grafts


Artificial skin grafts


“Smart” sutures

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Materials

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

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
H
H
H
H
H
H
H
H
H
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Materials

Electronics


Noncrystalline
silicon panels can
convert visible light
into electrical
energy.

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Materials

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

Liquid Crystals


Unlike liquids, molecules in liquid crystals
have some degree of order.

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Materials

Liquid Crystals


In
nematic liquid crystals
, molecules are only
ordered in one dimension, along the long
axis.

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Materials

Liquid Crystals


In
smectic liquid crystals
, molecules are
ordered in two dimensions, along the long
axis and in layers.

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Materials

Liquid Crystals


In
cholesteric liquid
crystals
, nematic
-
like crystals are
layered at angles to
each other.

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Materials

Liquid Crystals


These crystals can
exhibit color
changes with
changes in
temperature.

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Materials

Light
-
Emitting Diodes


In another type of
semiconductor, light
can be caused to be
emitted (LEDs).

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Materials

Light
-
Emitting Diodes (LEDs)


Organic light
-
emitting
diodes (OLEDs) are
lighter and more
flexible, and can be
brighter and more
energy efficient.


Soon OLEDs may
replace incandescent
lights in some
applications.

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Materials

Nanoparticles


Different
-
sized
particles of a
semiconductor (like
Cd
3
P
2
) can emit
different wavelengths
of light depending on
the size of the energy
gap between bands.

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Materials

Nanoparticles


Finely divided metals
can have quite different
properties than larger
samples of metals.

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Materials

Carbon Nanotubes


Carbon nanotubes
can be made with
metallic or
semiconducting
properties without
doping.