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Chapter 20, section 20.3Semiconductors and superconductors
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Metallic Bonds: The “electron” sea modelMetallic Bonds: The “electron” sea model
Valence electrons can move anywhere within the metalThese 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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1
Number ofatoms
e- inatoms
e- in latticeenergy levels
3
2
12
many
filled
empty
Formation of bands of valence orbitals in alkali metal
A metal behaves like a giant molecule
Valence orbitals extend over the entire metal
Electrons in partially filled valence band are free to move throughout the metal
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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 diamondbut 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 (+)
Electron (-)
Equivalent to
Which type of semiconductoris a better conductor? Why?
Analogy courtesy of Dr. Joe Lomax
N-type. Holes are less mobile than electrons
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Current flows in only one direction through p-n junction!
A A p-n p-n junction (rectifier)junction (rectifier). Charge carriers: holes (p-type) and e- (n-type)
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Solar panels employ a p-n junction
A photovoltaic cellphotovoltaic cell. Light drives e- around an external circuit
e-
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Metals are better conductors at low T.
Superconductors have zero resistance (are perfect conductors) at some (low) T.
Superconductors
typical metalR
esi
stan
ce
Temperature (K)
superconductor
Tc
critical T
0Wrong! Should approach
zero resistance!
Why are super conductors useful?
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Metal Tc (K)
Aluminum 1.15
Gallium 1.10
Tin 3.72
Mercury 4.15
Lanthanum 4.9
Lead 7.2
Several metals are superconductors at low T
Nb3Sn alloy 18.1
YBa2Cu3O7 90.
LaBa2Cu3Ox 35.
He(l) boils at 4.2 K
Many He(l)-cooled magnets use Nb-alloys
1st ceramicceramic superconductor (1986).
N2(l) boils at 77 K
“Y123” found 4 months later.
Highest Tc so far…Hg0.8Tl0.2Ba2Ca2Cu3O8.23 138.
Some alloys are better…
Superconductors
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Levitating train can travel at >300 mph!