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Chapter5_yy - Chapter 5: Diffusion in Solids ISSUES TO...

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Chapter 5 - 1 ISSUES TO ADDRESS. .. How does diffusion occur? Why is it an important part of processing? How can the rate of diffusion be predicted for some simple cases? How does diffusion depend on structure and temperature? Chapter 5: Diffusion in Solids
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Chapter 5 - 2 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Solids – vacancy diffusion or interstitial diffusion
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Chapter 5 - 3 Experiment: Pennies from different Years 2000 1994 1989
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Chapter 5 - 4 Interdiffusion : In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Adapted from Figs. 5.1 and 5.2, Callister 7e . Diffusion After some time
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Chapter 5 - 5 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D A B C D
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Chapter 5 - 6 Self-diffusion
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Chapter 5 - 7 Diffusion Mechanisms Vacancy Diffusion: atoms exchange with vacancies applies to substitutional impurity atoms rate depends on: --number of vacancies --activation energy to exchange. increasing elapsed time
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Chapter 5 - 8 • Simulation of interdiffusion across an interface: Rate of substitutional diffusion depends on: --vacancy concentration --frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation
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Chapter 5 - 9 Diffusion Mechanisms Interstitial diffusion – smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 5.3 (b), Callister 7e .
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Chapter 5 - 10
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Chapter 5 - 11 In order for atom to jump into a vacancy site, it needs to posses enough energy to break the bonds and squeeze through its neighbors. The energy necessary for motion - the activation energy Diffusion Atom Vacancy Q Distance Energy
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Chapter 5 - 12 Adapted from chapter-opening photograph, Chapter 5, Callister 7e. (Courtesy of Surface Division, Midland-Ross.) Case Hardening : --Diffuse carbon atoms into the host iron atoms at the surface. --Example of interstitial diffusion is a case hardened gear. • Result: The presence of C atoms makes iron (steel) harder. Processing Using Diffusion
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Chapter 5 - 13 Doping silicon with phosphorus for n -type semiconductors: • Process: 3. Result: Doped semiconductor regions. silicon Processing Using Diffusion magnified image of a computer chip 0.5 mm light regions: Si atoms light regions: Al atoms 2. Heat it. 1. Deposit P rich layers on surface. silicon Adapted from chapter-opening photograph, Chapter 18, Callister 7e.
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• Measured empirically - make thin film of known surface area - impose concentration gradient - measure how fast atoms or molecules diffuse through the membrane 12 Directional Quantity Flux can be measured for: --vacancies --host (A) atoms --impurity (B) atoms J x J y J z x y z x-direction Unit area A through which atoms move. Diffusion:
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This note was uploaded on 06/25/2008 for the course MSE 201 taught by Professor Brenner during the Spring '08 term at N.C. State.

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Chapter5_yy - Chapter 5: Diffusion in Solids ISSUES TO...

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