Transcript Band - Bryn Mawr College

Previously in
Chem 104:
• examples of
molecular solids
•Born Haber
Cycles
• “why doesn’t
that solid exist”
• phase
diagrams
•
TODAY
• Interchapter of
Modern Materials
•Band Theory
• and some Big Ideas
in the chapter
• Friday –
14.1, 14.2 & bring
your questions for
Recitation!
Big Idea 1.
Metals have Bonding “Bands”
How Band Theory Evolves from Molecular Orbital Theory
Recall the most basic view of MOT
Energy
antibonding orbital
atomic orbital,
Like 1s
atomic orbital,
Like 1s
bonding orbital
Make a little more complex:
Energy
2 antibonding MO’s
2 a.o.’s
2 a.o.’s
2 bonding MO’s
Make a lot more complex:
Energy
20 a.o.’s
20 antibonding MO’s
20 a.o.’s
20 bonding MO’s
Make a mole of a metal M:
Energy
6.022 x 1023 MO.’s:
a Band of
AntiBonding MO’s
6.022 x 1023 M a.o.’s:
make a Band
of many, many
closely
spaced
Atomic
orbitals
6.022 x 1023 a.o.’
6.022 x 1023 MO.’s:
a Band of Bonding MO’s
The Type of Element Determines Band Gap,
Band Gap = the energy separation between Bonding
and Antibonding Bands
Energy
AntiBonding
Band
Of a
Metal
Band Gap ~ 0 eV
Bonding
Band
Of a
Metal
The Type of Element Determines Band Gap
Energy
AntiBonding
Band
Of a
Metal
AntiBonding
Band
Of a
Network Solid
Band Gap is Large
Band Gap ~ 0 eV
Bonding
Band
Of a
Metal
Bonding
Band
Of a
Network Solid
~0 Band Gap Allows Electronic Movement
 makes Metal a Conductor
Energy
AntiBonding
Band
of a
Metal
is Empty
Band Gap ~ 0 eV
Bonding
Band
of a
Metal
is e- filled
Conduction
Band
e- ee- e-
e- ee- e-
Valence
Band
Large Band Gap Prevents Electronic Movement
 makes Metal an Insulator
Energy
Conduction Band
at High Energy
Band Gap is Too Large
for Electrons to “jump”
Valence
Band
At Low Energy
~Small Band Gap Allows Electronic Movement if
Energy added  makes a Semiconductor
Energy
Conduction
Band
by E = Light: Solar Cells
ee-
Band Gap overcome
by E = Heat: Thermisters
(heat regulators)
Valence
Band
Big Idea 3.
Impurities Create New Possibilties
~Impurities Decrease Band Gap
 makes a Better Semiconductor
Energy
Conduction
Band
Ge
eValence
Band
Ge
Ga doped –
a ptype
Ge
semiconductor
~Impurities Decrease Band Gap
 makes a Better Semiconductor
Energy
Conduction
Band
Ge
e-
Valence
Band
Ge
As doped –
an n-type
semiconductor
e-
Ge
Combining a P-type and N-type Semiconductors
Makes a Diode
N-type
P-type
e-
eee-
Current  this way only
A Diode made of the right materials causes DE loss to
be converted to Light: Light Emitting Diode (LED)
N-type
P-type
e-
e-
e-
The funny thing about corundum is, when you have it in a clean single
crystal, you get something much different.
Sapphire is Gem-quality corundum
Al2O3
with Ti(4+) & Fe(2+) replacing Al(3+)
Ruby
Gem-quality corundum Al2O3
with ~3% Cr(3+) replacing Al(3+)
Al2O3
Corundum
Al(3+): CN=6, Oh
O(2-): CN=4, Td
Nothing
recognizable here..
Big Idea 4.
Ceramics go beyond Dirt
Ceramics: can
The mean
Traditional
many View
things
Make from ground up rocks (“dirt”)
Composition: MAlxSiyOz.H2O
from silicate and aluminosilicate minerals
Begin “Plastic” (workable, malleable) when mixed with water
HEAT causes vitrification (“glassification”)
Structure: Amorphous with polycrystallites
or vitreous (glass)
Properties: very high melting points—refractories (furnace linings)
brittle (not malleable)
high mechanical strength and stability
chemically inert
Common example and how they differ:
From “common” clay; red color from
Terra cotta - FeO iron oxides in “dirt”
Fired at lowest temp; not glassy
Stoneware-
From “common” clay;
Fired at higher temp
Porcelain -
From flint + feldspar clays;
Fired at highest temp; more vitreous
China
Most translucent, most vitreous, most white, most pure
– Clay (kaolin) from China: Al2O3.2SiO2.2H2O .
“Bone China” originally made from calcined bone, CaO
The ‘ring’ test…
Firing process: evaporates remaining water away
and initiates vitrification
What goes on top of Ceramics
Is ceramic too — Glazes
Composition similar: silicates + flint + feldspar
(SiO2 + SiAlO3)
+ “flux” (K2O, ZnO, BaCO3
Structure: vitreous
Color from Transition Metal minerals/salts added
Fe(3+) – red-brown
Cu(2+) – turquoise blue and green
Co(2+) – “cobalt” blue
Ni(2+) – green, brown
Mn(2+) –purple, brown
Ceramics: the Modern View
Advanced Ceramics or Materials:
• silicon carbides SiC and nitrides Si3N
• composites: SiC/Al2O3 “whiskers”
Improved Properties:
• tougher, higher temperatures, fewer defects
Examples from Dr. Lukacs
• golf heads
• Machine parts
• tiles
All common stuff
Biggest Idea 5.
New Materials are Hot
Snazzy graphite relatives: fullerenes, carbon nantubes
drug delivery??
electronics?
Better materials for Solar cells
Superconducting solids
Molecular Magnets
Biomineralization: how does it grow like that?
Artificial bone?