ENGR 240 - 9.17.07

ENGR 240 - 9.17.07 - ENGR 240 September 17, 2007 Diffusion...

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ENGR 240 – September 17, 2007 Diffusion Mechanisms o For diffusion to take place An adjacent site must be empty Need enough energy to break the bond and cause the lattice distortion during the move Energy comes from vibrations (thermal energy) o Vacancy diffusion Moving from a normal lattice to a vacant one Occurs for self and inter-diffusion N v = Ne (-Qv/kT) As T increases, N v increases and vacancy diffusion increases o Interstitial diffusion Atoms move from one interstitial site to another Easily occurs with small atoms H, C, O, N Faster and more common than vacancy diffusion Small atoms tend to be more mobile and more sites are available Steady State Diffusion o Nothing changes with time o Diffusion flux (J) tells you how fast diffusion occurs J = m/At = mass of diffusing species/cross-sectional area*time o In differential form J = (1/A)(dm/dt) o Steady State flux does not change with time o Example: gas diffusing through a metal plate
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ENGR 240 - 9.17.07 - ENGR 240 September 17, 2007 Diffusion...

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