Transcript Liquids, Solids and Materials

Chapter 20, section 20.3
Semiconductors and superconductors
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Metallic Bonds: The “electron” sea model
Valence electrons can move anywhere within the metal
These electrons are delocalized (similar to resonance)
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To understand metallic bonding and electrical conductivity,
we need to re-visit molecular orbital theory.
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Formation of bands of valence orbitals in alkali metal
Number of
atoms
e- in
atoms
e- in lattice
energy levels
1
2
3
A metal behaves like a giant molecule
Valence orbitals extend
over the entire metal
12
empty
many
Electrons in partially filled valence band
are free to move throughout the metal
filled
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What if the valence band is full as in Mg?
Electrons can be excited to empty “conduction band”!
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So why don’t all solids conduct electricity?
Metals, Semiconductors & Insulators
Silicon and germanium form network solids similar to diamond
but they are semiconductors.
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Doped semiconductors: n-type and p-type
Si doped with As or P
Extra electron is all by itself
in the conduction band!
n-type semiconductor
Si doped with B
“hole” (positive charge)
conducts electricity!
p-type semiconductor
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Movement of holes requires concerted
movement of large numbers of electrons
Hole (+)
Equivalent to
Electron (-)
Which type of semiconductor
is a better conductor? Why?
N-type.
Holes are less mobile than electrons
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Analogy courtesy of Dr. Joe Lomax
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Current flows in only one direction through p-n junction!
A p-n junction (rectifier).
Charge carriers: holes (p-type) and e- (n-type)
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Solar panels employ a p-n junction
e-
A photovoltaic cell. Light drives e- around an external circuit
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Superconductors
Why are super
conductors useful?
Resistance
Metals are better conductors at low T.
critical T
Wrong! Should approach
zero resistance!
0
Tc
Temperature (K)
Superconductors have zero resistance (are perfect
conductors) at some (low) T.
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Superconductors
Several metals are superconductors at low T
Metal
Aluminum
Gallium
Tin
Mercury
Lanthanum
Lead
Nb3Sn alloy
LaBa2Cu3Ox
Tc (K)
1.15
1.10
3.72
4.15
4.9
7.2
18.1
35.
YBa2Cu3O7
90.
138.
Hg0.8Tl0.2Ba2Ca2Cu3O8.23
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Some alloys are better…
He(l) boils at 4.2 K
Many He(l)-cooled
magnets use Nb-alloys
1st ceramic
superconductor (1986).
N2(l) boils at 77 K
“Y123” found 4 months later.
Highest Tc so far…
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Levitating train can travel at >300 mph!
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