6.Diffusion%20in%20Solids - MSE 280: Engineering Materials...

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1 MSE 280: Engineering Materials Reading: Chapter 6 Diffusion in Solids ISSUES TO ADDRESS. .. How does diffusion occur? Why is it an important part of processing? • How can the rate of diffusion be predicted fo MSE280 © 2007-2009 Moonsub Shim, University of Illinois 1 How can the rate of diffusion be predicted for some simple cases? How does diffusion depend on structure and temperature? Diffusion Diffusion - Mass transport by atomic motion Mechanisms • Gases & Liquids – random (Brownian) motion • Solids – vacancy diffusion or interstitial diffusion Importance of diffusion? MSE280 © 2007-2009 Moonsub Shim, University of Illinois 2 All materials processing, Energy storage (e.g. fuel cells, batteries…), Filtration, Chemical reactions, Semiconductor devices, etc…
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2 PROCESSING USING DIFFUSION (1) Case Hardening : --Diffuse carbon atoms into the host iron atoms Fig. 5.0, Callister 6e . (Fig 5 0 is at the surface. --Example of interstitial diffusion is a case hardened gear. • Result: The "Case" is (Fig. 5.0 is courtesy of Surface Division, Midland- Ross.) MSE280 © 2007-2009 Moonsub Shim, University of Illinois 3 --hard to deform: C atoms "lock" planes from shearing . --hard to crack: C atoms put the surface in compression. From Cal ister 6e resource CD. PROCESSING USING DIFFUSION (2) Doping Silicon with P for n-type semiconductors: • Process: 0.5mm 1. Deposit P rich layers on surface. 2. Heat it. silicon magnified image of a computer chip Fig. 18.0, Callister 6e . MSE280 © 2007-2009 Moonsub Shim, University of Illinois 4 3. Result: Doped semiconductor regions. silicon light regions: Si atoms light regions: Al atoms From Cal ister 6e resource CD.
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3 DIFFUSION: THE PHENOMENON Interdiffusion : Atoms of one material diffusing into another and vice versa. e.g. In an alloy, atoms tend to migrate from regions of large concentration. Initially After some time 100% Cu Ni 100% Adapted from Figs. 5.1 and 5.2, Callister 6e . MSE280 © 2007-2009 Moonsub Shim, University of Illinois 5 Concentration Profiles 0 Concentration Profiles 0 From Cal ister 6e resource CD. Consider as a concentration gradient (atoms move from high conc. to low conc.). DIFFUSION: THE PHENOMENON Self-diffusion : Atoms within one material exchanging positions. (i.e. In an elemental solid, atoms also migrate). Label some atoms After some time A C A C D MSE280 © 2007-2009 Moonsub Shim, University of Illinois 6 B D B From Cal ister 6e resource CD.
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4 Diffusion mechanisms How do atoms move in a crystalline solid? For diffusion to occur For diffusion to occur: 1. Adjacent site needs to be empty (vacancy or interstitial). 2. Sufficient energy must be available to break bonds and overcome lattice distortion. There are different possible mechanisms but let’s MSE280 © 2007-2009 Moonsub Shim, University of Illinois 7 consider the simple/more common cases: 1. Vacancy diffusion.
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This note was uploaded on 03/15/2010 for the course MSE 280 taught by Professor Johnson during the Fall '08 term at University of Illinois, Urbana Champaign.

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6.Diffusion%20in%20Solids - MSE 280: Engineering Materials...

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