Transcript Chem 1201 - LSU Chemistry Home Page

Watkins
Chapter 12
CHAPTER 12
Modern Materials
• Liquid Crystals
• Polymers
• Ceramics
• Thin Films
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Liquid Crystals - Cholesterol Benzoate
Melting Point: 145C
Above 179C
Clear
Normal Liquid
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Between 145C and 179C
Milky
Liquid Crystalline
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Liquid Crystals
Three types
• Nematic
– 1-D order
• Smectic
– 2-D order
• Cholesteric
– 3-D order
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Liquid Crystals
Nematic and
Smectic molecules
are Rigid or Stiff,
Long and shaped
like Rods
Cholesteric molecules are
Flatter and have Flexible
Tails
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Liquid Crystals
Cholesteric liquid crystals
•Molecules are flat and sausage shaped with
flexible tails
•Changes in temperature, pressure or voltage
cause the ordering between layers to change.
•This results in color changes.
– Liquid Crystal Displays
– Liquid Crystal Thermometers
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Chapter 12
Polymers
Very Large Molecules
Composed of many smaller molecules (monomers).
Most familiar polymers (both natural and
synthetic) are organic compounds
Examples: plastics, DNA, proteins, rubber, starch,
cotton, wool, nylon
Other elements form inorganic polymers
Examples: Silicones, poly-phosphate minerals, polyphosphazines
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Polymers
• The monomer ethylene, C2H4, polymerizes to
form polyethylene
n C2H4
[C2H4]n
• This is called addition polymerization because
ethylene molecules are added to each other.
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Polymers
When some monomer molecules join together,
a small molecule (e.g. water) is also produced
O
H O
N H + H O C
N C
H
+ H O H
This is called condensation polymerization
Examples: nylon, proteins
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Polymers
Types of Synthetic Polymers
• Plastic: materials that can be formed
into shapes.
• Thermoplastic: materials that can be
shaped more than once using heat.
• Thermoset: materials that can only be
shaped once using heat.
• Elastomer: material that is elastic in
some way (rubber).
– Also useful for fibers.
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Polymers
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Polymers
Physical Properties of Polymers
In general, polymer chains are flexible and
easily entangled or folded
Each individual chain contains a different
number of monomer units
low density polyethylene (LDPE) has an average
molecular mass of 104 amu (~ 360 monomer
units, used in plastic wrap)
high density polyethylene (HDPE) has an average
molecular mass of 106 amu (~ 36,000 monomer
units, used in milk cartons)
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Polymers
Physical Properties of Polymers
Crystallinity (the amount of ordering in a
polymer) produces less flexibility
stiffer material, higher melting point
Stretching or extruding a polymer can increase
crystallinity
Degree of crystallinity generally increases with
average molecular mass
LDPE is more flexible and lower melting than HDPE
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Polymers
Crosslinking Polymers
• Natural rubber
– too soft, too chemically
reactive, not useful
• Crosslinking - bonds
formed between
polymer chains
– stiffer, less reactive
• Cross linking of rubber
is called vulcanization
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Polymers
Crosslinking Polymers
• Rubber is usually
cross-linked with
sulfur.
• Vulcanized rubber
is stiffer, more
elastic and less
susceptible to
chemical reaction.
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Ceramics
Inorganic (no C-H bonds)
Mostly metal oxides
Very high melting, hard, brittle
Less dense and more elastic than metals
Nonmetallic (insulator)
Crystalline or amorphous (glass-like) solids
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Ceramics
Common ceramics
Al2O3 (alumina, corundum)
SiO2 (sand, quartz, silica)
SiC (silicon carbide)
ZrO2 (zirconia)
BeO (beryllia)
YBa2Cu3O7 (YBCO or 123)
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Ceramics
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Ceramics
• Ceramic Composite: two or more different
ceramics mixed together
• Result: tougher ceramic.
• Most effective method: add fibers to ceramic
material. For example:
– SiC fibers added to aluminosilicate glass.
– Fiber must have a length 100 times its
diameter.
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Ceramics
Some Applications of Ceramics
• Cutting tools
– SiC drill bits, corundum sandpaper
• Electronics
– integrated circuits use alumina as insulator
• Piezoelectrics (generate a voltage during
mechanical stress)
– quartz watches, ultrasonic generators
• Thermal Tiles for the space shuttle.
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Ceramics
YBCO - A Hi-Tc Superconductor
• No electrical resistance
• Tc above LN2
• Meissner effect:
superconductor excludes all
magnetic field lines, so the
magnet floats in space
(maglev)
• Too brittle for wires, but
can make thin films
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Chapter 12
Thin Films
• Thin films generally have a thickness
between 0.1 m and 300 m.
• Useful thin films must
–
–
–
–
–
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be chemically stable,
adhere well to the surface,
be uniform,
be pure,
have low density of imperfections.
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Thin Films
Uses of Thin Films
• Microelectronics (conductors,
resistors and capacitors)
• Optical coatings
– reduce reflection from a lens
– Reduce scratching on lens
• Protective coatings for metals
• Increase hardness of tools
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Thin Films
Formation of Thin Film by Vacuum Deposition
Thin film material is placed in one vacuum chamber and
the objects to be coated (substrate) in another.
Examples
MgF2
Al2O3
SiO2
The material condenses on the substrate
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Thin Films
Formation of Thin Film by Sputtering
Thin Film material (M) is
placed on the negative
electrode
Substrate is the positive
electrode
Ar atoms (in the chamber) are
ionized to Ar+
Ar+ ions strike the negative
electrode and eject M.
Some M eventually hits the
substrate and condenses.
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Thin Films
Formation of Thin Film by CVD
Chemical Vapor Deposition
A volatile (gas phase) compound containing the thin
film metal reacts with another gas at the surface of the
substrate
The thin film material thus produced is very pure
TiBr4(g) + 2H2(g)  Ti(s) + 4HBr(g)
SiCl4(g) + 2H2(g)  Si(s) + 4HCl(g)
SiCl4(g) + 2H2(g) + 2CO2(g)  SiO2(s) + 4HCl(g) + 2CO(g)
3SiH4(g) + 4NH3(g)  Si3N4(s) + 12H2(g)
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