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ENGR 240 - 9.10.07

ENGR 240 - 9.10.07 - Self interstitials • atom stuck in...

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ENGR 240 – September 10, 2007 Imperfections in Solids (defects) o Defects can affect properties o Crystalline solids are always filled with millions of defects per cubic millimeter o Types of Defects Point defects (0-D) – atomic scale (one atom) Metals o Vacancy – empty lattice site All crystalline solids have vacancies Increase entropy of system Number increases as temperature increases “Thermally activated” process occurs as f(T) N v = N*exp(-Q v /kT) N v – number of vacancies N – number of atomic sites Q v – energy required to form a vacancy o Cu 0.9 eV/atom o Fe 1.08 eV/atom o Vacancy formation easier in Cu o Near melting point N v /N ~10 -4 k – Boltzmann constant o 1.38x10 -23 J/atom-K o 8.62x10 -5 eV/atom-K T – temperature in Kelvin Linear defects (1-D) – row of atoms Planar defects (2-D) – surface defect Volume defects (3-D) – holes, pores, voids
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Unformatted text preview: Self interstitials • atom stuck in space between 2 normal lattice sites • not so common, metals are closely packed • Ceramics o Contain both vacancies and interstitials o Must maintain charge balance o anions are large relative to cations anion interstitials are rare o cations form vacancies and interstitials o anions form only vacancies o Frenkel Defect cation interstitial and cation vacancy pair charge remains balanced a cation has moved/changed its location o Schottky Defect Anion and cation vacancy pair o Nonstoichiometric Stoichiometric - Ratio of cations and anions exactly match chemical formula May occur in ceramics when one ion can have multiple valence states Example: FeO in this state it is 2+ but can be 3+...
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