Chemistry, The Central Science, 11th edition
Theodore L. Brown, H. Eugene LeMay, Jr.,
and Bruce E. Bursten
Chapter 12
Modern Materials
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Materials Science
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Types of Materials
Recall that atomic
orbitals can be
combined linearly to
build molecular
orbitals. These
combinations lead to
bonding,
antibonding, and
non-bonding orbitals
(more than 3 atoms)
In such compounds, the energy gap between
molecular orbitals essentially disappears, and
continuous bands of energy states result.
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Band Theory
The Fermi Energy (Level) is the energy of the highest
occupied state.
Rather than having molecular orbitals separated by
an energy gap, these substances have energy
bands comprised of the closely spaced occupied
and empty orbitals.
conduction
band
valance
band
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The energy gap (Eg) between bands
determines whether a substance is a metal,
a semiconductor, or an insulator.
Eg is called the band gap
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The amount of orbital overlap will change the band gap
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Electronic properties of some materials
1 eV = 1.602 E-19 J
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Bond Polarity will influence Band Gap
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Some semiconducting
materials and
associated bandgaps
(eV)
12.14 For each of the following pairs of semiconductors,
which on will have the larger band gap?
a. InP or InAs
b. Ge or AlP
c. AgI or CdTe
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12.21 Silicon has a band gap of 1.1 eV at room
temperature. 1 eV = 1.602 E-19 J
a. What wavelength of light would be absorbed by crystalline silicon?
b. Does Si absorb some, all or what portion of sunlight?
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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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Doping (everyone does it)
By introducing very
small amounts of
impurities that have
more (n-Type) or
fewer (p-Type) valence
electrons, the
conductivity of a
semiconductor can be
increased.
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n-type semiconductors
result from the addition of
pentavalent impurities like
phosphorus, arsenic and
antimony. These donors
contribute “extra” electrons.
p-type semiconductors
result from the addition of
trivalent impurities like
boron, aluminum and
gallium. These additions
create valence “holes”
which act as additional
levels
Silicon has a diamond structrue with unit cell length 5.43 Å , and eight atoms per unit cell.
a) How many Si atoms are there in 1 cm3 of crystalline silicon?
b) Suppose you dope the 1 cm3 sample with 1 ppm of phosphorus to increase the conductivity. How many milligrams of phosphorus are required.
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How many atoms are there per
unit cell?
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http://britneyspears.ac/lasers.htm
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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Ceramics
Ceramics are made
from a suspension
of metal hydroxides
(called a sol).
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Ceramics
SEM micrograph of
amorphous Silica
particles. D ~200 nm
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Ceramics
Preparation of the
sol-gel begins by
reacting the metal
with an alcohol to
form an alkoxide.
Adding water results
in the fomation of the
M-OH.
The solution can then
be acidified to break
the O-Si-O bonds
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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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Superconductors
At very low
temperatures, some
substances lose
virtually all
resistance to the
flow of electrons.
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Superconductors
Much research has
been done recently
into the development
of high-temperature
superconductors…but
we seem to be losing
ground!.
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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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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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Addition Polymers
Addition polymers are made by coupling the
monomers by converting π-bonds within
each monomer to σ-bonds between
monomers.
Ethylene
Polyethylene
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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. Formation of a peptide bond
in proteins is an example
• These are also called copolymers.
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Synthesis of Nylon
Nylon (polyamides
is another example
of a condensation
polymer.
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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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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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Properties of Polymers
Such differences in
crystallinity can lead
to polymers of the
same substance
that have very
different physical
properties.
LDPE
HDPE
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Cross-Linking
Chemically bonding
chains of polymers
to each other can
stiffen and
strengthen the
substance.
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Cross-Linking
Naturally-occurring rubber is too soft and
pliable for many applications.
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Cross-Linking
Naturally-occurring rubber is too soft and
pliable for many applications.
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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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Recycling Plastic
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Recycling Plastic and the Ocean
Garbage Patch
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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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Biomaterials
• Biocompatibility
– The materials used
cannot cause
inflammatory
responses.
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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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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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Biomaterials
• These substances
are used to make:
– Heart valves
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Biomaterials
• These substances
are used to make:
– Heart valves
– Vascular grafts
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Biomaterials
• These substances
are used to make:
– Heart valves
– Vascular grafts
– Artificial skin grafts
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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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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.
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Electronics
Noncrystalline silicon
panels can convert
visible light into
electrical energy.
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Light-Emitting Diodes
In another type of
semiconductor, light
can be caused to be
emitted (LEDs).
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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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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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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, nematiclike crystals are
layered at angles to
each other.
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Liquid Crystals
These crystals can
exhibit color
changes with
changes in
temperature.
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Phase Diagrams for Liquid Crystals
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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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Ferromagnetic nanoparticles
made in Prof. Micheal Klem’s lab at Tech
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Nanoparticles
Finely divided metals
can have quite different
properties than larger
samples of metals.
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Carbon Nanotubes
Carbon nanotubes
can be made with
metallic or
semiconducting
properties without
doping.
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