Chapter 5 Flashcards
Bulk/Volume Diffusion
Motion of atoms through the body of the medium
Vacancy Diffusion
Interstitial Diffusion
Vacancy Diffusion
Atoms jump into vacancies.
Can move small or large atoms this way, including atoms of the same material as the medium itself
Interstitial Diffusion
Atoms jump into the interstitial spaces in between lattice locations.
Can usually only move small atoms this way
Surface Diffusion
motion of atoms along surfaces within the medium
Grain Boundary Diffusion
Phase Boundary Diffusion
Volume vs Surface Diffusion
Because atoms of the medium don’t line up properly at these surfaces, there is extra room for the diffusing species to fit and move. Therefore surface diffusion is much faster than bulk diffusion. On the other hand, there are millions of paths for the diffusing species to follow through the volume and only few for surfaces
Factors which increase the amount of diffusion
High Temperature
Long times
Small diffusing species
Medium through which diffusion occurs is not densely packed
Greater concentration gradients (larger differences in concentrations, smaller thickness of any diffusion barriers
Fick’s First Law
Steady State Diffusion
J = -D*(dc/dx) J = (Do)*exp(-Q/RT)
For use when the diffusion doesn’t change the concentration of the reservoirs appreciably
Note that the activation energy Q is the size of the energetic barrier that the atoms must overcome to move from it’s old location to its new location
In general Qvacancy > Qinterstitial because interstititals are smaller and do not need to nudge their neighbors far apart to move to the new location
Fick’s Second Law
Non-Steady State Diffusion
(dc/dt) = D(d2c / dx2)
For use when the concentration is changing as a function of time within the medium
Only solution to be aware of the 1-dimensional diffusion into a half plane with a fixed concentration resevoir
