miércoles, 26 de mayo de 2010

DIFFUSION: THE PHENOMENON

Why Study Diffusion ?

•Diffusion plays a crucial role in…
–Alloying metals => bronze, silver, gold
–Strengthening and heat treatment processes
•Hardening the surfaces of steel
–High temperature mechanical behavior
–Phase transformations
•Mass transport during FCC to BCC
–Environmental degradation
•Corrosion, etc.

How do atoms move in Solids ?Why do atoms move in Solids ?

•Diffusion, simply, is atoms moving from one lattice site to another in a stepwise manner
–Transport of material by moving atoms
•Two conditions are to be met:
–An empty adjacent site
–Enough energy to break bonds and cause lattice distortions during displacement
•What is the energy source ?
–HEAT !
•What else ?
–Concentration gradient !

Interdiffusion: in a solid with more than one type of element (an alloy), atoms tend to migrate from regions of large concentration.


Self-diffusion: In an elemental solid, atoms also migrate.


DIFFUSION MECHANISMS
Substitutional Diffusion:-applies to substitutional impurities-atoms exchange with vacancies-rate depends on: number of vacancies activation energy to exchange


DIFFUSION COUPLE MOVIE

Rate of substitutional diffusion depends on-vacancy concentration-frequency of jumping


Interstitial Diffusion:-applies to interstitial impurities-more rapid than vacancy diffusion PROCESSING USING DIFFUSION

Case Hardening
Diffuse carbon atoms into the host iron atoms at the surface. Interstitial diffusion

Result: The “Case” is -hard to deform C atoms “lock” planes from shearing
-hard to crack C atoms put surface in compression

Doping Silicon
With P for n-type semiconductors


Asignatura: EES
Fuente: www.matsceng.ohio-state.edu/mse205/.../chap5.pdf
Ver: http://diffusioninsolidsees.blogspot.com/

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