MSE2001H_4_2

MSE2001H_4_2 - 1 Chapter 4 Point Defects and Diffusion...

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1 • Introduction • Point Defects • Impurities • Solid State Diffusion Chapter 4 Point Defects and Diffusion
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2 Diffusion? Not Reversible (Non equilibrium) A process of mass transport from one part of a system to another as a result of random motions. A phenomenon of random motion causing a system to decay towards uniform conditions Once they reach an equilibrium state, the overall distribution of particle is not changed with time. Not Reversible (Non equilibrium)
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3 Example: Carburization of Steel Application of Diffusion surface core Extra carbon combines with iron to form strong iron carbide particles having excellent wear resistance. Æ The carburized surface is ideal for applications such as gears and crankshafts in automotive engines. Iron carbide (Fe 3 C)
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4 Self-Diffusion: diffusion without concentration gradient InterDiffusion: diffusion with concentration gradient Diffusion Initial stage After some time
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5 Fick’s First Law Factors affecting atomic transport: dc JD dx =− The net diffusion flux (J) J = D(C 1 –C 2 )/ Δ x Concentration gradient Jump distance, Δ x Structure Temperature A atoms B atoms D: the diffusion coefficient (cm 2 /s) Condition: concentration gradient is constant with time Fick’s First Law
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6 Question3) A thin plastic membrane is used to separate hydrogen from a gas stream. At steady state, the concentration of hydrogen on one side of the membrane is 0.025 mol/m 3 , on the other side it is 0.0025 mol/m 3 , and the membrane is 100 μ m thick.
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MSE2001H_4_2 - 1 Chapter 4 Point Defects and Diffusion...

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