Chapter 4 - Diffusion Flashcards
What does diffusion occur in?
- Gasses
- Liquids
- Solids
- Plasma
What is diffusion?
It is material transport by atomic motion
By diffusion inhomogeneous materials can become …
Homogeneous
What needs to happen for an active diffusion to occur?
The temperature needs to be high enough to overcome energy barriers to atomic motion.
When does inter diffusion occur?
in response to a concentration gradient
What is self diffusion?
This is diffusion in one component materials, when all atoms that exchange positions are of the same type
How does Vacancy Diffusion occur?
to jump from lattice site to lattice site, atoms need energy to break bonds with their neighbors, and to cause the necessary lattice distortions during the jump
What energy causes Vacancy diffusion?
Thermal Energy
What is the direction of the vacancy flow compared to the atomic flow?
Opposite
Why is interstitial diffusion faster than Vacancy diffusion?
Because the bonding of interstitials to surrounding atoms is weaker and there are more interstitial sites than there are vacancy sites.
What is used to quantify how fast diffusion occurs?
The diffusion flux
What is the flux defined as?
The number of atoms diffusing through unit area per unit time (atoms/m^2-secs)
or the mass of atoms diffusing through unit area per unit time (kg/m^2-secs)
J = M/At
What is Fick’s first law?
The diffusion flux along direction x is proportional to the concentration gradient.
J = -D (dC)/(dx)
- D = Diffusion coefficient
- dC = concentration gradient
- dx = change in x direction (distance/thickness)
What is the driving force also known as?
The composition gradient
What is Steady-State diffusion?
When the diffusion flux, J, does not change with time
What is the concentration profile?
The concentration of atoms/molecules of interest as a function of position
What is the concentration gradient?
the slope at a particular point on the concentration profile
What are the factors that affect diffusivity?
- Diffusion mechanism
- Type of crystalline structure of the matrix lattice (solvent)
- Type of imperfections or defects in the crystal
- Concentration of species at diffuse
- Temperature
Why would you dope silicon with phosphorus (P)?
To get an n-type semiconductor
What are the steps to doping?
- Deposit P rich layers on surface
- Heat it
- Result: Doped semiconductor regions
What does diffusion allow you to do in material processing?
Allows you to change the material properties in a controlled way, to suite the use for the material
What is Case Hardening?
Hardening of the surface of a metal by exposing it to impurities that will diffuse into the surface region and increase the surface hardness
E.G. Carburization of steel
