Chapter 12 Modern Materials - 中正大學化生系 - 國立中正大學

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Nov 1, 2013 (3 years and 7 months ago)

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材料化學與生物分子




從原子到宇宙

課程第八週


胡維平


國立中正大學化學暨生物化學系


11/08/2012



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Types of Materials


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



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Types of Materials


In very large clusters of
atoms, the energy gap
between molecular
orbitals essentially
disappears, and
continuous bands of
energy states result.



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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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Types of Materials


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



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Types of Materials



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Metals


Valence electrons are in a
partially
-
filled band.

Fe, Cu, Au, Ni


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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Semiconductors


Semiconductors have
a gap between the
valence band and
conduction band of
~50
-
300 kJ/mol.





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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


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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Insulators


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


They are not
conductive.



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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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Ceramics (
陶瓷材料
)



SiO
2

無機,非金屬性的固體材料


抗熱,抗壓,抗腐蝕,

重量輕,不變形


適合做機械元件


Al
2
O
3

SiC

Si
3
N
4

TiO
2

ZrO
2



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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.

(
許多為陶瓷材料
)



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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

(
高分子
)


Polymers

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

Some Common Polymers

聚苯乙烯

PS

聚氯乙烯

PVC

聚乙烯

PE

寶特

PET

保麗龍



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Addition Polymers

(
聚合高分子
)


Addition polymers

are made by coupling the
monomers by converting

-
bonds within each
monomer to

-
bonds between monomers.

Ethylene

Polyethylene



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Condensation Polymers

(
縮和高分子
)


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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Synthesis of Nylon


Nylon is one example of a condensation polymer.

n

H
2
N(CH
2
)
6
NH
2

+
n

HOOC(CH
2
)
4
COOH


+
n

H
2
O



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Properties of Polymers


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

PE



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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 (polymer of isoprene,
異戊二烯
)
is too soft and pliable for many applications.

In
vulcanization
,
chains are cross
-
linked by short chains of
sulfur

atoms,
making the rubber stronger and less susceptible to
degradation.

(Charles Goodyear, 1839)



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Biomaterials


Biocompatibility


The materials used cannot
cause inflammatory responses.


Physical Requirements


The properties of the material
must mimic the properties of
the “real” body part (i.e.,
flexibility, hardness, etc.).


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

Heart valves

using
Dacron
TM

Vascular grafts

using Dacron
TM


-
OCH
2
CH
2
OC(=O)PhC(=O)
-


Polyethylene terephthalate

(PET)

Artificial skin grafts

Using copolymer of glycolic acid
(
乙醇酸
)


and lactic acid

(
乳酸
)



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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


Noncrystalline silicon
panels can convert
visible light into
electrical energy.



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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


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


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



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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.

These crystals can exhibit color
changes with changes in
temperature.



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LCD

Display




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Light
-
Emitting Diodes


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



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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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Carbon Nanotubes


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



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Amino Acids and Proteins


Proteins are polymers of

-
amino acids.


A condensation reaction
between the amine end of one
amino acid and the acid end of
another produces a
peptide
bond
.

20
amino
acids



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Amino Acids and Proteins


Hydrogen bonding in peptide chains causes
coils and helices in the chain.



Kinking and folding of the coiled chain gives
proteins a characteristic shape.



Most enzymes are proteins.



The shape of the active site complements the
shape of the substrate on which the enzyme
acts
; hence, the “lock
-

and
-
key” model.




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Carbohydrates


Simple sugars are polyhydroxy
aldehydes or ketones. In solution,
they form cyclic structures.


Starch



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Nucleic Acids


Two of the building blocks of RNA
and DNA are sugars (ribose or
deoxyribose) and cyclic bases
(adenine, guanine, cytosine, and
thymine or uracil).



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1953

Watson and Crick

discovered the structure of
DNA and solve the mystery of
genetics



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DNA Replication

Protein Synthesis



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Origin of Life

Life on earth began ~35 byr ago.


Where did the water come from?

How were the biomolecules synthesized?

How did the first life begin?


Is the Universe fine
-
tuned for life?


From RNA World to DNA World


RNA can store information and can act
as an enzyme