Chapter 4 Slides

Chapter 4 Slides - Chapter 4 Lectures begin on Jan. 29, 08...

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1 Chapter 4 Chapter 4 Lectures begin on Jan. 29, 08 Modified slides 19, 20 on Feb. 18, 2008
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2 Chapter 4 Imperfections in Solids Chapter 4 Imperfections in Solids • What is a perfect solid?
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3 What are imperfections and why study them? What are imperfections and why study them? • Why study imperfections? – They are always present – They give us the ability to beneficially change the properties of a material • C added to Fe enormously strengthens it • Cr added to Fe enables us to protect against rust – All semiconductor technology depends making very pure Si single crystals, and then doping the Si with small amounts of P, As etc. to control the electrical conductivity – Mechanical properties of metals are controlled by the density of mobile dislocations – There are also BAD imperfections……. . • Imperfections occur at all dimensions (0D – point), 1D (line), 2D (interface or surface), and 3D (volume)
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4 Zero D defect Zero D defect a vacancy a vacancy Atoms can be imaged by the scanning tunneling microscope (STM) or the transmission electron microscope (TEM) At right we see an STM image of the (111) surface of Si with an atom missing – a vacancy We also see that atoms have distinct shapes and that Si (a Group IV, covalently bonded atom) is not spherical – consistent with the directional p orbitals involved in its bonding
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5 Outline of material to be covered Outline of material to be covered • 4.2 - Vacancies and self interstitials (point or 0D defects) • 4.3 - Impurities • 4.4 - Atomic per cent, weight per cent and volume per cent ( work the examples in the book by yourself ) • 4.5 - Dislocations – line defects • 4.6 - Interfacial defects – 2D defects • 4.7 – Bulk or volume defects – 3D • 4.8 – Atomic vibrations as measure of temperature • 4.10 – Microscopy (Light, electron, scanning probe) • 4.11 – Grain size determination Lecture concentrates on items in blue – and the material to be tested (except where notes as for at% and wt% conversions where it is left to you
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6 Vacancies : -vacant atomic sites in a structure. Self-Interstitials : -"extra" atoms positioned between atomic sites. Point Defects Point Defects Vacancy distortion of planes self- interstitial distortion of planes
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7 Point Defects Point Defects Substitutional: -Different atom in structure * Dopant atom is added intentionally * Impurity atom if not added intentionally Hetero-Interstitials : Different atom between the normal atoms in the structure
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8 (1.38 x 10 -23 J/atom-K) (8.62 x 10 -5 eV/atom-K) Boltzmann's constant ⎜⎟ N v N = exp Q v k T No. of defects No. of potential defect sites. Activation energy (+ or -???) Temperature in K Every lattice site is a potential vacancy site Defects are thermodynamically stable because they increase the entropy (randomness) of the system, decreasing the free energy • Equilibrium concentration varies with temperature • Q. Does concentration of defects increase or decrease with T ?
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Chapter 4 Slides - Chapter 4 Lectures begin on Jan. 29, 08...

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