Alloys and Solid SolutionsAlloys and Solid Solutions

Chemistry 123Spring 2008

Dr. Woodward

SolutionsSolutions

Solid Solution14 Carat Gold

Liquid SolutionVodka

Gaseous Solution

Air

Solution = A homogeneous mixture

AlloysAlloysAn alloy is a material that contains more than one element

and has the characteristic properties of a metal.

Primary Primary ElementElement

Name of AlloyName of Alloy Composition Composition

(by mass)(by mass)PropertiesProperties

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

Substitutional (Homogeneous) Substitutional (Homogeneous) AlloysAlloys

Substitutional AlloySubstitutional Alloy

Two or more types of metal atoms are randomly distributed over the positions occupied in

the host metal.

Atoms should be similar in size (as a rule of thumb, atomic radii should not differ by more than

15%)

Interstitial AlloyInterstitial Alloy

Smaller atoms (typically nonmetals) occupy some of the

holes or interstitial positions in the lattice.

The smaller nonmetal atoms typically bond covalently with the metal atoms, which increases the

hardness and strength (but reduces the ductility)

14-Karat Gold Steel

Au

Ag

Fe

C

Other Types of AlloysOther Types of Alloys

Intermetallic CompoundsIntermetallic Compounds

Intermetallic compounds are not solutions. They have a fixed composition (just like

molecular substances) with well defined properties.

Some examples include Ni3Al which is a strong lightweight alloy used in aircraft engines, and Co5Sm, which is used to

make magnets.

Heterogeneous AlloyHeterogeneous Alloy

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.

Pearlite shown here is a mixture of essentially pure iron and the

binary phase, Fe3C.

Ni3Al Superalloy Pearlite

Al

Ni

Fe Metal Fe3C

+ +

Semiconductor Solid SolutionsSemiconductor Solid Solutions

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.

Heterojunction laser

CdS-CdSe Solid SolutionsCdS-CdSe Solid SolutionsCdS1-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 (ECdS (Egg = 2.4 = 2.4 eV)eV)

CdSe (ECdSe (Egg = 1.7 = 1.7 eV)eV)

Cadmium pigmentsCadmium pigments

CdSCdS1-x1-xSeSexx compositionscompositions

Band Gap vs. CompositionBand Gap vs. CompositionHow 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):

g g

g g

E A E Intx

E A E B

ExampleExample

What composition in the CdS1-

xSex solid solution will have a band gap of 2.25 eV?

What color will this compound be?

SolutionThe 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.

2.4 2.250.21

2.4 1.7x

The Color WheelThe Color Wheel

UV 100-400 nm 12.4 - 3.10 eVViolet 400-425 nm 3.10 - 2.92 eVBlue 425-492 nm 2.92 - 2.52 eVGreen 492-575 nm 2.52 - 2.15 eVYellow 575-585 nm 2.15 - 2.12 eVOrange 585-647 nm 2.12 - 1.92 eVRed 647-700 nm 1.92 - 1.77 eVNear IR 10,000-700 nm1.77 - 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 orangeorange.

UV-Visible SpectraUV-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.