Presentation on theme: "CH-4: Imperfections in Solids"— Presentation transcript:
1 CH-4: Imperfections in Solids 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:Catalytic ConvertersMolecules 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.
2 Catalyst: (Ce0.5Zr0.5)O2Catalyst 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.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.
3 Catalysts and Surface Defects A catalyst increases the rate of a chemical reaction without being consumedActive sites on catalysts are normally surface defectsFig. 4.10, Callister & Rethwisch 8e.Single crystals of (Ce0.5Zr0.5)O2 used in an automotive catalytic converterFig. 4.11, Callister & Rethwisch 8e.
5 Point Defects in Metals • Vacancies:-vacant atomic sites in a structure.Vacancydistortionof planes• Self-Interstitials:-"extra" atoms positioned between atomic sites.self-interstitialdistortionof planes
6 Equilibrium Concentration: Point Defects • Equilibrium concentration varies with temperature!No. of defectsActivation energyNæQövv=expçNo. of potentialNèkTødefect sitesTemperatureBoltzmann's constant-23(1.38 x 10J/atom-K)-5(8.62x10eV/atom-K)Each lattice siteis a potentialvacancy site
7 Measuring Activation Energy æçNv=exp-QkTèöø• We can get Qv froman experiment.• Measure this...NvTexponentialdependence!defect concentration• Replot it...1/TNvln-Q/kslope
8 Estimating Vacancy Concentration • Find the equil. # of vacancies in 1 m3 of Cu at 1000C.• Given:r= 8.4 g/cm3A= 63.5 g/molCuQ= 0.9 eV/atomN= 6.02 x 1023atoms/molvA= 2.7 x 10-48.62 x 10-5eV/atom-K0.9 eV/atom1273 KçNv=exp-QkTæèöøFor 1 m3, N =NACurx1 m3= 8.0 x 1028 sites• Answer:Nv=(2.7 x 10-4)(8.0 x 1028) sites = 2.2 x 1025 vacancies
9 Impurities in SolidsA pure metal consisting of only one type of atom just isn’t possible. Even with sophisticated techniques, it is difficult to refine metals to a purity in excess of %.Very few metals are used in the pure or nearly pure state: 1. Electronic wires % purity Cu; Very high electrical conductivity.% 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.Cartridge brass: 70% Cu & 30% Zn.Sterling silver: 92.5% Ag & 7.5% Cu.Inconel 718, Ni-base super-alloy, used for jet engine parts, has 10 elements.
10 Solid Solutions Simplest type of alloy is that of solid solution. Two types: 1. Substitution Solid Solution2. Interstitial Solid Solution.
11 Conditions for Solid Solubility Conditions for substitutional solid solution (S.S.)W. Hume – Rothery rule1. r (atomic radius) < 15%2. Proximity in periodic tablei.e., similar electronegativities3. Same crystal structure for pure metals4. ValencyAll else being equal, a metal will have a greater tendency to dissolve a metal of higher valency than one of lower valency
12 Application of Hume–Rothery rules – Solid Solutions Element Atomic Crystal Electro- Valence Radius Structure nega- (nm) tivityCu FCC C H O Ag FCC Al FCC Co HCP Cr BCC Fe BCC Ni FCC Pd FCC Zn HCP4.4: 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 solutionTable on p. 118, Callister & Rethwisch 8e.