Chapter-5 - 9/12/11 1 0 Chapter 5 Imperfections in Solids...

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Unformatted text preview: 9/12/11 1 0 Chapter 5 Imperfections in Solids Learning Objectives Classify, describe, and illustrate imperfections in crystal structures Classify methods of examining defects 1 A scanning probe micrograph (generated using a scanning- tunneling microscope) that shows a (111)-type surface plane* for silicon. The arrow points to the location of a silicon atom that was removed using a tungsten nanotip probe. This site from which an atom is missing is the surface analogue of a vacancy defect that is, a vacant lattice site within the bulk material. Approximately 20,000,000 . (Micrograph courtesy of D. Huang, Stanford University.) Reasons to Study Imperfections Materials properties heavily influenced by the presence of imperfections include: Optical Electronic Mechanical (strength) Chemical 9/12/11 2 2 5-2 Point Defects in Metals Point defects are local disruptions of the crystal lattice. Although they may be defined in terms of a single lattice site, points defects structurally influence neighboring sites. In a compositionally pure material, the simplest defects consist of self-interstitials or vacancies. Vacancies are many more times prevalent than self-interstitials. 3 5-2 Vacancies and Self-Interstitials exp v v Q N N kT = Vacancies arise from the absence of an atom from the crystal structure. The density of vacancy structures is temperature dependent. N v =# vacancies/cm 3 N = # atoms/cm 3 Q v = energy required to create 1 mol vacancies k = Boltzmann ` s constant T = temperature (K) 9/12/11 2 2 5-2 Point Defects in Metals Point defects are local disruptions of the crystal lattice. Although they may be defined in terms of a single lattice site, points defects structurally influence neighboring sites. In a compositionally pure material, the simplest defects consist of self-interstitials or vacancies. Vacancies are many more times prevalent than self-interstitials. 3 5-2 Vacancies and Self-Interstitials exp v v Q N N kT = Vacancies arise from the absence of an atom from the crystal structure....
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Chapter-5 - 9/12/11 1 0 Chapter 5 Imperfections in Solids...

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