CH-4: Imperfections in Solids So far we have seen perfect crystals: X-ray diffraction and Braggs law Imperfections or defects are covered in ch-4. Defects in crystals make them interesting 2 major types of defects: Chemical Impurities or Alloying elements Atomic arrangement- Structure Real materials are not perfect Good Chemical Imperfections: dopants & alloying element Atomic arrangements: missing atom, extra atom, differently oriented unit cells, etc Why STUDY Imperfections in Solids? Many of the important properties of materials are due to the presence of imperfections. Pure metals experience significant alterations when alloyed: Sterling silver: 92.5% Ag & 7.5% Cu. Cartridge brass: 70% Cu & 30% Zn.
Impurities play important roles in semiconductors. Steel (composition ) and (making) Atomic defects are responsible for reducing gas pollutant emissions in automobiles: Molecules of pollutant gases become attached to surface defects of crystalline metallic materials ((Ce0.5Zr0.5)O2) in the catalytic converter. While attached to these sites, chemical reactions convert them into other non- or less-polluting substances. Catalyst: (Ce0.5Zr0.5)O2 Catalyst is a substance that speeds up the rate of a chemical reaction without participating in the reaction itself. Catalyst adsorbs on its surface gas pollutants (CO and NOX) and molecules of unburned hydrocarbons, which are converted to CO2 and H2O. Active sites on catalysts are normally surface defects Schematic representation of surface
defects that are potential adsorption sites for catalysts. High-resolution transmission electron micrograph of single crystal (Ce0.5Zr0.5)O2,which is used in Catalytic Converters. http://auto.howstuffworks.com/cat alytic-converter2.htm In the catalytic converter, there are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as the materials are extremely expensive. The catalyst used in a catalytic converter is a combination of platinum (Pt), palladium (Pd), and rhodium (Rh). These metals coat a ceramic honeycomb (or ceramic beads) contained within a metal casing that is attached to the exhaust pipe. The catalytic converters honeycomb structure provides the maximum surface area on which reactions can take place while using the least amount of catalyst. - See more at:
http://www.explorecuriocity.org/Content.aspx? contentid=1779#sthash.ygSRJfFB.dpuf Types of Imperfections Vacancy atoms Interstitial atoms Substitutional atoms Point defects Dislocations Line defects Grain Boundaries Area defects 5 Point Defects in Metals
Vacancies: -vacant atomic sites in a structure. Vacancy distortion of planes Self-Interstitials: -"extra" atoms positioned between atomic sites. selfinterstitial distortion of planes 6 Equilibrium Concentration: Point Defects Equilibrium concentration varies with temperature! Activation energy No. of defects
No. of potential defect sites Qv Nv = exp kT N Temperature Boltzmann's constant -23 (1.38 x 10 (8.62x 10 -5 J/atom-K) eV/atom-K) Each lattice site
is a potential vacancy site 7 Measuring Activation Energy Qv Nv = exp kT N We can get Qv from an experiment. Measure this... Replot it... Nv ln N
Nv slope N - Q v /k exponential dependence! T 1/ T defect concentration 8 Estimating Vacancy Concentration Find the equil. # of vacancies in 1 m3 of Cu at 1000C. Given: = 8.4 g / cm 3 r
Q v A N = 0.9 eV/atom Cu A = 63.5 g/mol = 6.02 x 1023 atoms/mol 0.9 eV/atom Qv Nv = = 2.7 x 10-4
exp kT N 1273 K For 1 m 3 ,N= r x N A 8.62 x 10-5 A x 1 m3 eV/atom-K = 8.0 x 1028 sites
Cu Answer: Nv = (2.7 x 10-4)(8.0 x 1028) sites = 2.2 x 1025 vacancies 9 4.1 Calculate the fraction of atom sites that are vacant for lead at its melting temperature of 327C (600 K). Assume an energy for vacancy formation of 0.55 eV/atom. 4.3 Calculate the activation energy for vacancy formation in aluminum, given that the equilibrium number of vacancies at 500C (773 K) is 7.57 1023 m-3. The atomic weight and density (at 500C) for aluminum are, respectively, 26.98 g/mol and 2.62 g/cm3. Impurities in Solids A pure metal consisting of only one type of atom just isnt possible. Even with
sophisticated techniques, it is difficult to refine metals to a purity in excess of 99.9999%. Very few metals are used in the pure or nearly pure state: 1. Electronic wires- 99.99% purity Cu; Very high electrical conductivity. 2. 99.99% purity Al (super-pure Al) is used for decorative purposes-- Very bright metallic surface finish. Most engineering metals are combined with other metals or nonmetals to provide increased strength, higher corrosion resistance, etc. 1. Cartridge brass: 70% Cu & 30% Zn. 2. Sterling silver: 92.5% Ag & 7.5% Cu. 3. Inconel 718, Ni-base super-alloy, used for jet engine parts, has 10 elements. Solid Solutions Simplest type of alloy is that of solid solution. Two types: 1. Substitution Solid Solution 2. Interstitial Solid Solution. Conditions for Solid Solubility Conditions for substitutional solid solution (S.S.) W. Hume Rothery rule 1. r (atomic radius) < 15% 2. Proximity in periodic table
i.e., similar electronegativities 3. Same crystal structure for pure metals 4. Same Valency An example of a substitutional solid solution is found for copper and nickel. These two elements are completely soluble in one another at all proportions. The atomic radii for copper and nickel are 0.128 and 0.125 nm, respectively; both have the FCC crystal structure; their electronegativities are 1.9 and 1.8 and their valences match. 13 Application of HumeRothery rules Solid Solutions Element 4.4: Which of these elements would you expect to form the following with copper: (a) A substitutional solid solution having complete solubility (b) A substitutional solid solution of incomplete solubility
(c) An interstitial solid solution Cu C H O Ag Al Co Cr Fe Ni Pd Zn Pt Atomic Crystal Radius Structure (nm) 0.1278 0.071 0.046
+3 +2 +2 +2 +2 +2 14 Interstitial Solid Solutions Computation of Radius of BCC Octahedral Interstitial Site Compute the radius r of an impurity atom that just fits into a BCC octahedral site in terms of the atomic radius R of the host atom (without introducing lattice strains). Computation of Radius of FCC Octahedral Interstitial Site Compute the radius r of an impurity atom that just fits into a FCC octahedral site in terms of the atomic radius R of the host atom (without introducing lattice strains).
Computation of Radius of BCC Tetrahedral Interstitial Site Compute the radius r of an impurity atom that just fits into a BCC tetrahedral interstitial site in terms of the atomic radius R of the host atom (without introducing lattice strains).
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