Lecture 2 6pp - Imperfections: Good or Bad? Structural...

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1 Imperfections: Good or Bad? • Structural imperfections (defects) • Compositional imperfections (impurities) 2 Structural Imperfections • A perfect crystal has the lowest internal energy E • Above absolute zero at constant volume, the system responds by minimizing the free energy F = E - TS, where T is the absolute temperature and S entropy 3 Structural Imperfections E –TS F = E – TS Defect concentration Equilibrium defect concentration 4 Compositional Imperfections • Impurities always present in materials • Without impurities, semiconductor devices and advanced engineering materials not possible • Si devices cannot function without low- level impurities • Silver not useable as utensils without addition of copper 5 Solid Solutions • Consider addition of Ni into Cu (both FCC) • Ni atoms occupy Cu sites • Substitutional solid solution • Rules of miscibility – Atomic size – Crystal structure – Electronegativity – (Valence) 6 Ni/Cu • Atomic size: 0.125 nm vs 0.128 nm • Crystal structure: both FCC • Electronegativity: 1.91 vs 1.90 • Valence: both +2 Fully miscible
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7 Zn/Cu • Atomic size: 0.139 nm vs 0.128 nm • Crystal structure: HCP vs FCC • Electronegativity: 1.65 vs 1.90 • Valence: both +2 Cu can only dissolve 38 at. % Zn 8 Solid Solutions • Consider addition of Ni into Cu (both FCC) • Ni atoms occupy Cu sites • Substitutional solid solution • Rules of miscibility – Atomic size – Crystal structure – Electronegativity – (Valence) Minimize elastic strain Minimize elastic strain Minimize chemical reactivity and possibly elastic strain 9 Two Types of Solid Solutions • Substitutional - impurity or solute atoms occupying regular lattice sites, e.g., Ni in Cu • Interstitial - impurity or solute atoms occupying “holes” between lattice sites, e.g., C in Fe – Applicable for small atoms, e.g., C – Low solubility
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This note was uploaded on 11/21/2011 for the course MAT SCI 201 taught by Professor Matsci during the Spring '10 term at Northwestern.

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Lecture 2 6pp - Imperfections: Good or Bad? Structural...

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