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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= Boltzmanns 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 /A X N A = Avogadros 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 dont 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 /(A tot ) o Q fr = energy required for formation of each frenkel defect o k=Boltzmanns constant o T= Temperature AX-type compound equilibrium number for Schottky defect...
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- Spring '10