Alloys and Solid Solutions Chemistry 123 Spring 2008 Dr. Woodward

Solutions Solid Solution 14 Carat Gold Liquid Solution 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, Au14-Carat Gold58% Au, 42% Ag Harder than pure gold Copper, CuYellow Brass67% Cu, 33% Zn Ductile, Takes a polish Iron, FeStainless Steel80.6% Fe, 18% Cr, 1% Ni, 0.4% C Harder and more corrosion resistant than pure iron Lead, PbPlumber’s Solder67% Pb, 33% Sn Low melting point (275 °C) Silver, AgSterling Silver92.5% Ag, 7.5% Cu Bright surface that is more inert than pure silver

Substitutional (Homogeneous) Alloys Substitutional 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 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 GoldSteel Au Ag Fe C

Other Types of Alloys Intermetallic Compounds Intermetallic compounds are not solutions. They have a fixed composition (just like molecular substances) with well defined properties. Some examples include Ni 3 Al which is a strong lightweight alloy used in aircraft engines, and Co 5 Sm, which is used to make magnets. Heterogeneous 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, Fe 3 C. Ni 3 Al SuperalloyPearlite Al Ni Fe MetalFe 3 C +

Semiconductor Solid Solutions Figures taken from “Semiconductor Optoelectronic Devices”, by P. Bhattacharya The solid solution between GaAs (Band gap, E g =1.4 eV, Unit cell edge, a=5.65 Å) and AlAs (Band gap, E g =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 Solutions CdS 1-x Se x solid solutions are excellent pigments (cadmium yellow, cadmium orange). By controlling the composition we can control the band gap and hence the color. CdS (E g = 2.4 eV) CdS (E g = 2.4 eV) CdSe (E g = 1.7 eV) CdSe (E g = 1.7 eV) Cadmium pigments CdS 1-x Se x compositions CdS 1-x Se x compositions

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 E g (int) with composition A 1-x B x can be estimated from the band gaps of the end members, E g (A) and E g (B):

Example What composition in the CdS 1-x Se x 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.

The Color Wheel UV nm eV Violet nm eV Blue nm eV Green nm eV Yellow nm eV Orange nm eV Red nm eV Near IR 10, nm 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 o oo orange.

UV-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.