5-DIFFUSION-CHPT 5-MIAE 221-2023

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Concordia University *

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221

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Material Science

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Oct 30, 2023

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1 DIFFUSION - ATOM MOVEMENT IN MATERIALS Chapter 5 MIAE 221 – Materials Science

Topic# 5A - MIAE 221 (adapted from Callister) 2 DIFFUSION - ATOM MOVEMENT IN MATERIALS Movement of atoms in SOLIDS (+ liquids, gases) is very IMPORTANT. Hardening steel - Chrome-plating - Gas reactions - Si wafers - Ceramic sintering…..

Page_140b Topic# 5A - MIAE 221 (adapted from Callister) 3

Topic# 5A - MIAE 221 (adapted from Callister) 4 RATES – reaction rates are governed by activation energies In order for atoms to move (in a solid) or to react, atoms need sufficient energy. ACTIVATION ENERGY : Q Q is the extra energy (above average energy) needed to overcome energy barrier to allow atom movement. Energy supplied by heat . So as T increases , number of atoms having sufficient energy to move or react also increases . This type of expression holds true for many reaction rates. They are known as ARRHENIUS Reaction rates.

Topic# 5A - MIAE 221 (adapted from Callister) 5       − = RT Q C Rate exp 0 where C 0 = constant Q = activation energy (J.mol -1 ) T = temperature, K (Note: 0 K = -273 o C) R = gas constant (8.314 J.mol -1 .K -1 ) Take natural logs of both sides:       − = RT Q C Rate ) ln( ) ln( 0 So plot graph of ln Rate # 1/T to get straight line, slope = -Q/R (This type of rate relationship holds for many reactions) ARRHENIUS Reaction rates

Topic# 5A - MIAE 221 (adapted from Callister) 6 DIFFUSION MECHANISMS Movement of Matter Through Matter by atomic motion GASES - quick movement LIQUID - Slower (ink etc) SOLIDS - Much slower In solids, atoms are in fixed positions so movement is more difficult than liquid or gas. Atoms vibrate so some movement possible (above absolute zero). For atoms to diffuse in solids, require : • an empty adjacent site (e.g. vacancy) • sufficient energy to break bonds with neighbours and squeeze through gap .

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