ENGR 220 Lecture 9 Chapter 4 All Except 4.11
Crystal Wall Gazing Reading
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Microscopic Examination A. Crystallites (Grains) 1. Vary considerably in size 2. Can be quite large a) Up to several cm b) Large single crystals of quartz, diamond or Si c) Galvanized railings (1) Can easily see the grains B. Optical Microscopy 1. Useful up to only 1000-2000X magnification 2. Polishing removes surface scratches 3. Chemical etching changes reflectance a) Depending on crystal orientation 4. Grain boundaries a) Area imperfections b) Variation in crystal orientation across boundaries c) More susceptible to chemical etching d) Dark lines 5. Polarized Light C. Scanning Electron Microscopy 1. Surface scanned by an electron beam and reflected electrons collected to produce an image 2. Surface must be electrically conductive D. Transmission Electron Microscopy 1. Electron beam passes through sample 2. Image created due to contrast from beam 3. Sample must be very thin <100nm a) Magnification up to 1,000,000X
ENGR 220
Lecture 9 Chapter 5
Diffusion A. What is Diffusion 1. Mass transport a) Movement of actual material via atomic motion B. Mechanisms 1. Gases and Liquids a) Colored food dye in water b) Smoke particles under a microscope (1) Brownian Motion (a) Random motion observed (2) Explained by einstein he;ped prove the existence of atoms and molecules 2. Solids a) Vacancy diffusion (1) Interstitial diffusion C. Interdiffusion 1. Alloys a) Atoms tend to migrate from regions of high concentration to regions of low concentration D. Self-Diffusion 1. E. Vacancy Diffusion 1. Atoms exchanging places with vacancies 2. Applies to substitutional impurity atoms 3. Rate depends on a) Number of vacancies b) Activation energy for exchange Diffusion Mechanisms A. Vacancy B. Interstitial
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