Alloys and Solid Solutions
Chemistry 123
Spring 2008
Dr. Woodward
Solutions
Solid Solution
Liquid Solution
14 Carat Gold
Vodka
Gaseous Solution
Air
Solution = A homogeneous mixture
Alloys
An alloy is a material that contains more than one
element and has the characteristic properties of a metal.
Primary
Element
Name of Alloy
Composition
(by mass)
Properties
Gold, Au
14-Carat Gold
58% Au, 42% Ag
Harder than pure gold
Copper, Cu
Yellow Brass
67% Cu, 33% Zn
Ductile, Takes a polish
Iron, Fe
Stainless Steel
80.6% Fe, 18% Cr,
1% Ni, 0.4% C
Harder and more
corrosion resistant
than pure iron
Lead, Pb
Plumber’s Solder
67% Pb, 33% Sn
Low melting point
(275 °C)
Silver, Ag
Sterling Silver
92.5% Ag, 7.5% Cu
Bright surface that is
more inert than pure
silver
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Substitutional (Homogeneous) Alloys
14-Karat Gold
Steel
Fe
Au
C
Ag
Substitutional Alloy
Interstitial Alloy
Two or more types of metal atoms
are randomly distributed over the
positions occupied in the host
metal.
Smaller atoms (typically nonmetals)
occupy some of the holes or
interstitial positions in the lattice.
Atoms should be similar in size (as
a rule of thumb, atomic radii
should not differ by more than 15%)
The smaller nonmetal atoms typically
bond covalently with the metal atoms,
which increases the hardness and
strength (but reduces the ductility)
Other Types of Alloys
Ni3Al Superalloy
Pearlite
Fe Metal
Al
Fe3C
+
Ni
Intermetallic Compounds
Heterogeneous Alloy
Intermetallic compounds are not
solutions. They have a fixed
composition (just like molecular
substances) with well defined
properties.
A heterogeneous alloy is not
homogeneous. It consists of two or
more distinct phases, each with its
own composition. The properties are
sensitive to the way a sample was
made.
Some examples include Ni3Al which
is a strong lightweight alloy used in
aircraft engines, and Co5Sm, which
is used to make magnets.
Pearlite shown here is a mixture of
essentially pure iron and the binary
phase, Fe3C.
Semiconductor Solid Solutions
Heterojunction laser
Figures taken from “Semiconductor
Optoelectronic Devices”, by P. Bhattacharya
The solid solution between GaAs (Band gap, Eg=1.4 eV, Unit cell edge, a=5.65
Å) and AlAs (Band gap, Eg=2.1 eV, Unit cell edge, a=5.66 Å) is among the most
important for optoelectronic devices. By forming solid solutions we can control
the band gap.
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CdSCdS-CdSe Solid Solutions
CdS1-xSex solid solutions are excellent pigments (cadmium yellow, cadmium
orange). By controlling the composition we can control the band gap and hence
the color.
CdS (Eg = 2.4 eV)
eV)
CdS1-xSex compositions
CdSe (Eg = 1.7 eV)
eV)
Cadmium pigments
Band Gap vs. Composition
How does the band gap vary as we change the composition across a
solid solution?
For semiconductors it is not unusual that the band gap will vary
(approximately) linearly as the composition changes.
In such cases the band gap of an
intermediate composition Eg(int) with
composition A1-xBx can be estimated
from the band gaps of the end
members, Eg(A) and Eg(B):
x=
Eg ( A ) − Eg ( Int )
Eg ( A) − Eg ( B )
Example
What composition in the CdS1-xSex
solid solution will have a band gap
of 2.25 eV?
What color will this compound be?
Solution
The end member with the larger band
gap is A=CdS, while the end member
with the smaller band gap is B=CdSe.
Using their band gaps we can
calculate x that will give a band gap
of 2.25 eV.
x=
2.4 − 2.25
= 0.21
2.4 − 1.7
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The Color Wheel
UV
Violet
Blue
Green
Yellow
Orange
Red
Near IR
100-400 nm
400-425 nm
425-492 nm
492-575 nm
575-585 nm
585-647 nm
647-700 nm
10,000-700 nm
12.4 - 3.10 eV
3.10 - 2.92 eV
2.92 - 2.52 eV
2.52 - 2.15 eV
2.15 - 2.12 eV
2.12 - 1.92 eV
1.92 - 1.77 eV
1.77 - 0.12 eV
A semiconductor with a band gap of 2.25 eV will absorb all visible
light with energy greater than 2.25 eV. This means it will absorb
the violet, the blue and most of the green.
The reflected colors will be red, orange and yellow. Therefore,
the color will be orange.
orange
UVUV-Visible Spectra
This plot shows UV-Visible spectra for CdS, CdSe and ZnS. Based
on what you know about the colors of these compounds identify which
curve goes with which compound.
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