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Unformatted text preview: Chapter 5 Imperfections in Solids Introduction • All crystalline materials on an atomic scale have imperfections • Crystalline defect= a lattice irregularity having one or more of its dimensions on the order of an atomic diameter • Classification of crystalline imperfection based on type of defect • Defects: point defects (associated with one or two atomic positions), linear defects (one dimensional), interfacial defects (boundaries or two dimensional) Point Defects in Metals • Vacancy= simplest of the point defects • All crystalline solids have vacancies • Number of vacancies, N V = N*exp(-Q V /(kT)) o N= number of atomic sites o Q V = energy required for the formation of a vacancy o T= absolute temperature in Kelvin o K= Boltzmann’s constant= 1.38*10-23 J/atom-K • Number of vacancies increases with temperature • Self-interstitial = an atom from the crystal that is crowded into an interstitial site, a small void space that under ordinary circumstances is not occupied o Formation is not highly probable o Exists in very small concentrations o In metals large distortions • N, Number of atoms per unit volume for metal o N=N A ρ/A X N A = Avogadro’s number A x = atomic weight of metal Point Defects in Ceramics • Ceramics also have vacancies and interstitials • Defects can occur for each ion in the ceramic • Defect structure = used to designate the types and concentrations of atomic defects in ceramics • When defect structures are considered conditions of electroneutrality must be maintained • Electroneutrality = the state that exists when there are equal numbers of positive and negative charges from the ions • As a consequence defects don’t occur alone • Frenkel defect = one type of defect involves a cation-vacancy and a cation-interstitial pair o No change in charge o Cation leaving its normal position an moving into an interstitial site • Schottky defect = a cation vacancy-anion vacancy pair defect found in AX materials o Removing one cation and one anion from the interior of the crystal and then placing both them at an external surface • Stoichiometry = defined as a state for ionic compounds wherein there is the exact ratio of cations to anions predicted by the chemical formula • Number of cation-vacancy/cation-interstitial Frenkel defect pairs, N fr =N*exp(-Q fr /(2kT)) o N= number of lattice sites =N A ρ/(A tot ) o Q fr = energy required for formation of each frenkel defect o k=Boltzmann’s constant o T= Temperature • AX-type compound equilibrium number for Schottky defect...
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This note was uploaded on 01/02/2012 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.
- Spring '10