Chapter-6 - 9/22/11 Chapter 6 Diffusion 6-1 Introduction...

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9/22/11 1 Copyright © 2006 by Nelson, a division of Thomson Canada Limited 5-0 6-1 Introduction Diffusion can be described as material transport by atomic motion. Chapter 6 Diffusion Copyright © 2006 by Nelson, a division of Thomson Canada Limited 6-1 6-1 Introduction- Diffusion Heat treatment of steels (carburization and nitridation) for enhanced hardness Doping of semiconductors (modification of conductivity) Conductive Ceramics (fuel cells) Permeation barriers (plastic packaging) Protective coatings (chemically passivating coatings, thermal barrier coatings) NOTE: Although we commonly think of diffusion in systems involving gases and liquids, it is an equally important topic/characteristic within systems entailing solids. (See Example 5-1)
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9/22/11 2 Copyright © 2006 by Nelson, a division of Thomson Canada Limited 6-2 Copyright © 2006 by Nelson, a division of Thomson Canada Limited 6-3 6-2 Mechanisms for Diffusion The diffusion of atoms or ions within a solid can occur via a number of different mechanisms. As suggested in chapter 4, the presence of defects can significantly alter both the mechanism and rate of diffusion. The driving force behind diffusion is best understood in thermodynamic terms, where the movement of species serves to increase the entropy of the system, thus lowering the free energy of the system. Examples of differing mechanisms include self-diffusion, interdiffusion, vacancy diffusion and interstitial diffusion. G H T S Δ = Δ − Δ
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9/22/11 3 Copyright © 2006 by Nelson, a division of Thomson Canada Limited 6-4 Activation Energy for Diffusion Thermal energy serves to supply the energy required to overcome the activation energy and accomplish diffusion.
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Chapter-6 - 9/22/11 Chapter 6 Diffusion 6-1 Introduction...

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