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Diffusion Forsiden Universitetet i Oslo扩散forsiden universitetet我奥斯陆
KIFEE symposium Trondheim Sept. 9 2006 KJM5120 and KJM9120 Defects and Reactions Truls Norby Diffusion – important property! Diffusion describes transport of species (particles, atoms, ions, molecules) through a medium (gas, liquid, solid) Here we consider only solid-state diffusion Diffusion in crystalline solids takes place only by defects; in a perfect structure, there can be no diffusion Many essential processes take place by solid-state diffusion: Reactions, synthesis, mixing, doping, permeation, ionic conduction, sintering, grain growth, creep, corrosion, a.o. These comprise the synthesis, fabrication, utilisation, and breakdown of materials Diffusion – many terms There are many terms of diffusion Random diffusion Self diffusion Defect diffusion Tracer diffusion Chemical diffusion Ambipolar diffusion Random diffusion is caused only by thermal energy and does not require any gradients or driving force Self diffusion reflects random diffusion of a component (e.g. oxide ions) Defect diffusion reflects random diffusion of a defect The remaining terms reflect diffusion with a net transport in some direction, driven by a gradient – a driving force However, also these gradient-driven diffusion phenomena are governed by random, thermally activated diffusion! Diffusion - knowing defects brings insight Diffusion is considered a difficult theme, because of the many terms and how they are interlinked. The difficulties arise especially in the presence of driving forces. Diffusion coefficients in driving forces are often termed chemical diffusion coefficients, and without understanding of defects, they may remain obscure and phenomenological. However, by understanding defects, it is possible to get control of it all. This is actually one of the main goals of the course. In this Ch 5. we mostly deal with random, thermally driven diffusion. In Ch. 6 we deal with diffusion of charged species under an electrical field, i.e. electrical conduction. In Ch. 7 we deal
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