Lecture 11 - Ch 5 pt 1

Lecture 11 - Ch 5 pt 1 - Chapter 5: Diffusion in Solids...

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Unformatted text preview: Chapter 5: 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 for some simple cases? How does diffusion depend on structure and temperature? Chapter 5 - 1 ANNOUNCEMENTS #1 Reading: Chapter 5, all sections Recitation Problems: #s 5.3, 5.7, 5.13, 5.16, 5.22, 5.25 This topic will be included in Midterm I Midterm I: 8:00 am, Tuesday May 4, Main Auditorium Be there or…;-) Chapter 5 - 2 ANNOUNCEMENTS #2 Midterm Coverage: Material from Chapters 1, 2, 3, 4 & 5 Exam Format: 3 Questions (100 Pts.): o 1 short answer, multi-part…choose/circle the correct answer(s); write in a few words or numbers; show brief calculation… o 2 longer answer, 2-3 part…calculation + explanation…similar to the recitation problems Formula Sheet & Periodic Table provided Work smartly & efficiently…don’t write essays, show full working… Accommodations: LeBow 348; 8:00-9:15 (1.5X) or 8:00-9:40 am (2X) Chapter 5 - 3 Diffusion What is Diffusion?: Mass transport (i.e. movement of actual material) via atomic motion Mechanisms: Gases & Liquids: o random (Brownian) motion Solids: o vacancy diffusion or interstitial diffusion Chapter 5 - 4 Diffusion • Interdiffusion: In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. After some time t Initially Adapted from Figs. 5.1 and 5.2, Callister 7e. Chapter 5 - 5 Diffusion • Self-diffusion: In an elemental solid, atoms can also migrate… Let’s label some atoms After some time t C C A D B D A B Chapter 5 - 6 Diffusion Mechanisms • Vacancy Diffusion: atoms exchange with vacancies applies to substitutional impurity atoms rate depends on: - number of vacancies - activation energy for exchange to occur Increasing elapsed time Chapter 5 - 7 Diffusion Mechanisms - 2 types Vacancy Interstitial Chapter 5 - 8 Diffusion Simulation • Simulation of interdiffusion across an interface • Rate of substitutional diffusion depends on: - vacancy concentration - frequency of jumping (Courtesy P.M. Anderson) Chapter 5 - 9 Diffusion Mechanisms • Interstitial diffusion: smaller solute atoms can diffuse between larger solvent atoms Adapted from Fig. 5.3 (b), Callister 7e. More rapid than vacancy diffusion…Why? Chapter 5 - 10 Diffusion • How do we quantify the amount, or rate, of diffusion? J Flux moles (or mass) diffusing mol kg = or 2 (surface area )(time ) cm s m2s • Measured empirically: – Make thin film (membrane) of known surface area – Impose a concentration gradient – Measure how fast atoms or molecules diffuse through the membrane M= M 1 dM J slope mass J= = diffused At A dt time Chapter 5 - 11 Diffusion - 2 Cases • Steady-State Diffusion: – Concentration profile IS NOT a function of time • Non-steady State (or Transient) Diffusion: – Concentration profile IS a function of time Chapter 5 - 12 ...
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This note was uploaded on 07/27/2011 for the course ENGR 134 taught by Professor Marks during the Spring '11 term at Drexel.

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