Lecture 57 The Solid State II

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Lecture 57 - The Solid State II
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Review

• Unit Cell Coordination Particles in
• Number Unit Cell
• Simple cubic 6 1
• BCC 8 2
• FCC 12 4
• Hexagonal 12 4

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Ionic Solids - e.g. NaCl
Cl- ions are FCC
Na ions fit in spaces between Cl-
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Ionic Solids - e.g. CuCl
Cl- ions are FCC
Cu ions surrounded tetrahedrally by Cl-
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Metallic Solids

• high Tm strong bonds
• malleability non-directional bonds
• Electron Sea Model

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Electron Sea Model
mobile electrons conduct heat and electricity
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Band Model, e.g. Mg
Empty MOs
3p
Filled MOs
3s2
2p6
2s2
1s2
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Band Model, e.g. Mg
electrons in conduction band are free to move
e-
e-
e-
e-
apply an electric potential...
e-
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Alloys

• - contain a mixture of substances and have
  metallic properties
• - can be substitutional or interstitial

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Substitutional Alloys
Cu
Zn
Brass 2/3 Cu, 1/3 Zn
Sterling silver 93 Ag, 7 Cu Pewter 85 Sn, 7
Cu, 6 Bi, 2 Sb Solder 67 Pb, 33 Sn
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Interstitial Alloys
Fe
C
Steel Fe, lt2 C
Iron - relatively soft, ductile, malleable Steel
- hard, strong, less malleable (strong
directional bonds of C)
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Properties Vary With Composition

• Steel C Properties Uses
• mild lt0.2 malleable nails
• medium 0.2-0.6 harder beams
• high carbon 0.6-1.5 very hard tools

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Other Alloys of iron

• Fe (C, Si, Mn, Mo, Cr)
• Chromoly Steel, high strength/weight ratio
• Fe (C, Mn, Si, Cr, Ni)
• Stainless Steel

more resistant to oxidation
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Diamond - A Covalent Network Solid
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.
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C
.
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.
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C - C - C
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.
.
.
.
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Diamond - sp3 hybrid bonds
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Graphite
sp2 hybrid bonds
4th valence e-
movement possible
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MOs in network solids
Empty MOs
Energy
Empty MOs
Empty MOs
Filled MOs
Filled MOs
Filled MOs
Graphite
Metals
Diamond
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Bonding vs. Properties

• Property Diamond Graphite
• hardness high low
• conductivity low high
• slipperiness low high

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Si in network solids

• in biological systems, C-C-C-C-C
• in nature, Si-Si-Si-Si-Si-Si-Si??

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Si in network solids

• O C O

similar sizes
C sp hybrid p bond (C 2p)
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Si in network solids

• O Si O ?

poor overlap with Si 3p orbitals
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Silica, SiO2
O tetrahedrally about Si
strong covalent bonds, High Tm
repeating unit is SiO2
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Molecular Solids

• - lattice positions occupied by molecules
• - strong intramolecular forces,
• weaker intermolecular forces
• - thus, relatively low Tm
• e.g. ice, dry ice, P4, S8

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The Special Case of Ice

• liquid water - almost random arrangement of
  molecules
• ice - hexagonal arrangement of molecules, less
  dense

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Semiconductors
Si small energy gap some conduction
Si 1 As extra electrons
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n-type semiconductor
.
Si Si As Si
.
As Si Si Si
.
Si Si As Si
electrons easily enter conduction band and
carry charge
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p-type semiconductor
Si Si B Si

B Si Si Si


Si Si B Si
charge carriers are holes
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p-n junction
(-)
()
forward bias - current flows!
()
(-)
reverse bias - no current flows!