Diffusion Flashcards
In what phases can diffusion occur?
Gasses, liquids, solids, plasma
Define diffusion
Material transport by atomic motion
What inhibits diffusion in solids?
The temperature must be high enough to overcome energy barriers to atomic motion
What is self-diffusion?
All atoms exchanging position are of the same element.
What is inter-diffusion (impurity diffusion)?
The movement of distinct elements across a concentration gradient
What is the mechanism of vacancy diffusion?
Atoms use thermal energy to break bonds between neighbouring atoms to jump to a vacancy.
What is the vacancy flow direction opposite to?
The flow of atoms
What is the mechanism of interstitial diffusion?
Again, interstitial atoms use thermal energy to break bonds with neighbours. However the bounds are weaker and there are more vacancies therefore the diffusion occurs quicker.
What is the restriction on interstitial diffusion?
Atomic size
Which atoms can commonly diffuse interstitially?
Hydrogen, Carbon, and Oxygen
small impurity atoms
What is the measure of diffusion?
Diffusion Flux, J (used
to quantify how fast diffusion occurs)
What is the unit of diffusion flux?
atoms/m^2s or kg/m^2s
What is Fick’s first law?
the diffusion flux along direction x is
proportional to the concentration gradient
J= - D * dC/dx (m^2/s)
What is a factor which influence diffusivity? (1)
- Atom size
- The type of crystalline structure must match
- Imperfections. surface defects leads to grain boundary defects and that leads to volume defects
- Concentration of solute;
When the concentration of the solute increases, the diffusion is modified - Temperature;
As temperature increases, diffusing increases
What is doping?
Purposeful diffusion
Essential for all semiconductors
Doping silicon with phosphorous for n-type semiconductors
Why do we dope materials?
Alter properties for a specific application
What is case hardening?
Hardening the surface of a material by allowing impurities to diffuse into the surface and increase surface hardness
How does diffusion effect inhomogeneous materials?
Inhomogeneous materials can become homogeneous by diffusion
What is steady state diffusion?
the diffusion flux J does not change with time
What is a common example of case hardening?
Carburisation of steel. Diffusion of carbon atoms
(interstitial mechanism) increases concentration of C atoms and makes iron (steel) harder.
