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
