Chemistry
Third Edition
Julia Burdge
Lecture PowerPoints
Chapter 12
Modern Materials
Copyright © 2012, The McGraw-Hill Compaies, Inc. Permission required for reproduction or display.
CHAPTER
12.1
12.2
12.3
12.4
12.5
12.6
12.7
12
Modern Materials
Polymers
Ceramics and Composite Materials
Liquid Crystals
Biomedical Materials
Nanotechnology
Semiconductors
Superconductors
2
12.1
Polymers
Topics
Addition Polymers
Condensation Polymers
3
12.1
Polymers
Addition Polymers
Many biological molecules, such as DNA, starch, and proteins,
have very large molecular masses.
These molecules are polymers because they are made up of
many smaller parts linked together.
The small molecules that make up the individual building
blocks of polymers are called monomers.
4
12.1
Polymers
Addition Polymers
Some of these polymers are thermoplastic, which means that
they can be melted and reshaped, or heated and bent.
Others are thermosetting, which means that their shape is
determined as part of the chemical process that formed the
polymer.
Thermosetting polymers cannot be reshaped easily and are
not easily recycled, whereas thermoplastic polymers can be
melted down and cast into new shapes in different products.
5
12.1
Polymers
Addition Polymers
The simplest type of polymerization reaction involves the
bonding of monomer molecules by movement of electrons
from a multiple bond into new single bonds between
molecules.
This type of polymerization is called addition polymerization.
6
12.1
Polymers
Addition Polymers
Polyethylene
7
12.1
Polymers
Addition Polymers
© David Tietz/Editorial Image, LLC
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12.1
Polymers
Addition Polymers
9
12.1
Polymers
Addition Polymers
10
12.1
Polymers
Addition Polymers
11
12.1
Polymers
Addition Polymers
12
12.1
Polymers
Addition Polymers
Copolymers are polymers made from two or more different
monomers.
13
SAMPLE PROBLEM
12.1
Polypropylene is produced by the addition polymerization of
propylene, H2C=CH-CH3.
Draw the structure of polypropylene, showing at least two
repeating monomer units, and write its general formula.
14
SAMPLE PROBLEM
12.1
Setup
15
SAMPLE PROBLEM
12.1
Solution
16
12.1
Polymers
Addition Polymers
(a) isotactic, (b) syndiotactic, and (c) atactic polymers.
17
12.1
Polymers
Condensation Polymers
Condensation polymers form when monomers with two
different functional groups combine, resulting in the
elimination of a small molecule (usually water).
18
12.1
Polymers
Condensation Polymers
Polyamides
19
12.1
Polymers
Condensation Polymers
Polyesters
20
SAMPLE PROBLEM
12.2
Kevlar, a condensation polymer used in bulletproof vests, has
the following general structure:
Write the structure of the two monomers that form Kevlar.
21
SAMPLE PROBLEM
12.2
Solution
22
SAMPLE PROBLEM
12.3
Propyne (HCCCH3) can be used to form an electrically
conducting polymer. Draw the structure of polypropyne,
showing at least three repeating units, and write the general
formula for the polymer.
Setup
23
SAMPLE PROBLEM
12.3
Solution
24
12.2
Ceramics and Composite Materials
Topics
Ceramics
Composite Materials
25
12.2
Ceramics and Composite Materials
Ceramics
The use of ceramics in the form of pottery dates back to
antiquity.
All these ceramics are polymeric inorganic compounds that
share the properties of hardness, strength, and high melting
points.
Ceramics are usually formed by melting and then solidifying
inorganic substances (including clays).
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12.2
Ceramics and Composite Materials
Ceramics
Ceramics can be prepared by heating a slurry of a powder of
the inorganic substance in water to a very high temperature
under high pressure.
This process, called sintering, bonds the particles to each
other, thus producing the finished ceramic.
27
12.2
Ceramics and Composite Materials
Ceramics
The sol-gel process is frequently employed for modern
ceramics that are used in structural applications.
The sol-gel process produces particles of nearly uniform size
that are much more likely to produce a solid ceramic without
gaps or cracks.
28
12.2
Ceramics and Composite Materials
Ceramics
The first step in the sol-gel process is the preparation of an
alkoxide of the metal or metalloid that is going to be made
into the ceramic.
29
12.2
Ceramics and Composite Materials
Ceramics
The suspension of the metal oxide-hydroxide polymer is
called a gel.
The gel is carefully heated to remove the liquid, and what
remains is a collection of tiny, remarkably uniformly sized
particles.
Sintering of material produced by the sol-gel process
produces a ceramic with relatively few imperfections.
30
12.2
Ceramics and Composite Materials
Composite Materials
A composite material is made from two or more substances
with different properties that remain separate in the bulk
material.
Each contributes properties to the overall material, though,
such that the composite exhibits the best properties of each
of its components.
31
12.3
Liquid Crystals
Topics
Liquid Crystals
32
12.3
Liquid Crystals
Liquid Crystals
Liquid crystals are substances that exhibit properties of both
liquids, such as the ability to flow and to take on the shape of
a container, and those of crystals, such as a regular
arrangement of particles in a lattice.
Liquids are isotropic, because their properties are
independent of the direction of testing.
Liquid crystals are anisotropic because the properties they
display depend on the direction (orientation) of the
measurement.
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12.3
Liquid Crystals
Liquid Crystals
The structure of cholesteryl benzoate is fairly rigid due to the
presence of the fused rings and sp2-hybridized carbon atoms.
The molecule is relatively long compared to its width, too.
34
12.3
Liquid Crystals
Liquid Crystals
Rigidity and this particular shape makes it possible for the
cholesteryl benzoate molecules to arrange themselves in an
orderly manner, in much the way that pencils, chopsticks, or
tongue depressors can be arranged.
35
12.3
Liquid Crystals
Liquid Crystals
(a) Nematic, (b) cholesteric, and (c) smectic liquid crystals.
36
12.3
Liquid Crystals
37
SAMPLE PROBLEM
12.4
Would the following molecule make a good liquid crystal?
Why or why not?
Strategy
Examine the structure in question to see if it has rigid regions
and to see if it is longer in one dimension than in another.
38
SAMPLE PROBLEM
12.4
Setup
Carbon atoms that are sp2-hybridized contribute to the
rigidity of a molecule’s shape.
Solution
The molecule meets the structural requirements and should
make a good candidate for liquid crystal behavior.
39
12.4
Biomedical Materials
Topics
Dental Implants
Soft Tissue Materials
Artificial Joints
40
12.4
Biomedical Materials
Dental Implants
Many modern fillings are made of dental amalgam, a solution
of several metals in mercury.
Modern dental amalgam consists of 50 percent mercury and
50 percent of an alloy powder that usually contains (in order
of abundance) silver, tin, copper, and zinc.
These metals tend to expand slightly with age, causing
fissures and cracks in the tooth that may require further
intervention (e.g., crowns, root canals, or tooth replacement).
41
12.4
Biomedical Materials
Soft Tissue Materials
Artificial Skin and Sutures
42
12.4
Biomedical Materials
Artificial Joints
Polymethyl methacrylate
(PMMA) and polyethylene
have been used in many total
joint replacements.
© Jim Wehtje/Photodisc/Getty
43
12.5
Nanotechnology
Topics
Graphite, Buckyballs, and Nanotubes
44
12.5
Nanotechnology
Graphite, Buckyballs, and Nanotubes
Nanotechnology is the development and study of such smallscale materials and objects.
Graphene
45
12.5
Nanotechnology
Graphite, Buckyballs, and Nanotubes
C60, The Buckyball
© Getty Royalty Free
buckminsterfullerene
46
12.5
Nanotechnology
Graphite, Buckyballs, and Nanotubes
Carbon Nanotubes
47
12.6
Semiconductors
Topics
Semiconductors
48
12.6
Semiconductors
Semiconductors
The band energies and the gaps (or lack of gaps) between the
bonding band (called the valence band) and the antibonding
band (called the conduction band) make it possible to classify
a substance as an electrical conductor, a semiconductor, or an
electrical insulator based on the behavior of the electrons in
the bands.
49
12.6
Semiconductors
Semiconductors
50
12.6
Semiconductors
Semiconductors
Silicon, germanium, and carbon in the form of graphite are
the only elemental semiconductors at room temperature.
All three of these elements are in Group 4A and have four
valence electrons. (Tin and lead are also in Group 4A, but they
are metals, not semiconductors.)
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12.6
Semiconductors
Semiconductors
Other semiconductors consist of combinations of elements
whose valence electron count totals 8.
For example, gallium (Group 3A) and phosphorus (Group 5A)
form a semiconductor because gallium contributes three
valence electrons and phosphorus contributes five, giving a
total of eight valence electrons.
Semiconductors have also been formed between Group 2B
elements (particularly Zn and Cd) and Group 6A elements.
52
SAMPLE PROBLEM
12.5
State whether each of the following combinations of
elements could form a semiconductor:
(a) Ga-Se
(b) In-P
(c) Cd-Te
53
SAMPLE PROBLEM
12.5
Solution
(a) Ga has three valence electrons, and Se has six. This gives a
total of nine, which is too many to form a semiconductor.
(b) In (three valence electrons) and P (five valence electrons)
combine for a total of eight, so In-P should be a
semiconductor.
(c) Cd (two valence electrons) and Te (six valence electrons)
also combine for a total of eight, so Cd-Te should be a
semiconductor, too.
54
12.6
Semiconductors
Semiconductors
The conductivity of a semiconductor can be enhanced greatly
by doping, the addition of very small quantities of an element
with one more or one fewer valence electron than the natural
semiconductor.
55
12.6
Semiconductors
Semiconductors
Silicon is a Group 4A element, so it has four valence electrons per
atom. A small (parts per million) amount of phosphorus (Group
5A, five valence electrons) can be added, thus doping the silicon
with phosphorus.
Since each phosphorus atom has an “extra” electron relative to the
pure semiconductor, these extra electrons must reside in the
conduction band, where they increase its conductivity.
This type of doped semiconductor is called an n-type
semiconductor because the semiconductivity has been enhanced
by the addition of negative particles, the extra electrons.
56
12.6
Semiconductors
Semiconductors
A silicon semiconductor could be doped with small amounts
(again, parts per million) of gallium (Group 3A, three valence
electrons).
Instead of an “extra” electron, there is a “hole” (one less
electron) in every place that a gallium atom has replaced a
silicon atom.
These holes are effectively positive charges (because each is
the absence of an electron), so this type of material is called a
p-type semiconductor.
57
12.6
Semiconductors
Semiconductors
A p-type semiconductor has increased conductivity because
the holes (which are in the valence band) move through the
silicon rather than electrons.
The energy required to move an electron from the valence
band into a hole (also in the valence band) is considerably less
than the energy needed to promote an electron from the
valence band to the conduction band of a semiconductor.
Thus, the movement of the holes results from the movement
of charge, and conductivity is the movement of charge.
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12.6
Semiconductors
Semiconductors
59
12.6
Semiconductors
Semiconductors
Diodes, which are electronic devices that restrict the flow of
electrons in a circuit to one direction, work in essentially the
opposite direction from solar cells.
A light-emitting diode (LED) consists of n-type
and p-type semiconductor layers placed in contact.
When a small voltage is applied, the “extra” electrons from
the n-type side combine with the holes in the p-type side,
thus emitting energy (light) whose wavelength depends on
the band gap.
60
12.7
Superconductors
Topics
Superconductors
61
12.7
Superconductors
Superconductors
Superconductors have no resistance to the flow of electrons
and thus could be very useful for the transmission of
electricity over the long distances between power plants and
cities and towns.
Mercury is a superconductor at 4 K, the temperature of liquid
helium. Liquid helium is very expensive, however, so there
were few feasible applications of a mercury superconductor.
62
12.7
Superconductors
Superconductors
Many superconductors have been discovered including a
lanthanum-, barium-, copper-, and oxygen-containing ceramic
compound, and a series of copper oxides (“cuprates”), all of
which become superconducting below 77 K, the temperature
of liquid nitrogen (a much more common and less expensive
refrigerant than liquid helium).
The temperature below which an element, compound, or
material becomes superconducting is called the
superconducting transition temperature, Tc. The higher the
Tc, the more useful the superconductor.
63
12.7
Superconductors
Superconductors
YBCO – A High-Temperature Superconductor
64
12.7
Superconductors
Superconductors
The Meissner Effect
© Bill Pierce//Time Life Pictures/Getty
Images
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