MCE 230_F10_lec_17 - MCE 230 Materials Science Fall 2010...

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MCE 230 – Materials Science Fall 2010 Lecture - 17
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Diffusion
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Analysis of Diffusion Process Diffusion is a time-dependent process Diffusion Flux : number of atoms (or mass) that is crossing a given area per unit time. t A M J J diffusion flux (kg/m 2 –s) or (atoms/m 2 –s) M no. of diffusing atoms (or mass of diffusing atoms) A area across which diffusion is occurring t diffusing time Steady state diffusion J is not changing with time Non-steady state diffusion J is changing with time
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Steady State Diffusion Fick’s first law Diffusion flux is proportional to concentration gradient (concentration gradient is the driving force for diffusion). x C D J D: diffusion coefficient (m 2 /s) Negative sign indicate that diffusion is down the concentration gradient (from high to low)
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Non-Steady State Diffusion If J is changing with time non-steady state Concentration gradient varies with position AND time 2 2 x C D t C Fick’s second law:
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This note was uploaded on 12/05/2010 for the course MECHANICAL mce 230 taught by Professor Bassildrass during the Fall '10 term at American University of Sharjah.

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MCE 230_F10_lec_17 - MCE 230 Materials Science Fall 2010...

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