Chapter 5 Review Flashcards
Diffusion refers to the net flux of any species, such as ions, atoms, electrons, holes, and molecules.
What are the applications of diffusion?
HEAT TREATMENT OF METALS
*Carburization for surface hardening of steel.
*Dopant diffusion for semiconductor devices
*Conductive ceramics
*Creation of plastic beverage bottles
*Oxidation of aluminum
*Coatings and thin films
*Thermal barrier coatings for turbine blades
*Optical fibers and microelectronic components
*HEAT TREATMENT OF METALS
What types of diffusion exist?
Volume Diffusion: Movement of atoms within the bulk of a material, driven by thermal energy
Grain Boundary Diffusion: Faster diffusion along the interfaces between grains in a polycrystalline material.
Surface Diffusion: Movement of atoms along the surface of a material
What are the mechanisms of diffusion?
Which ones occur more frequently?
- Vacancy diffusion: The ‘hole’ moves around
Typically has higher activation energy (happens less often) - Interstitial diffusion: interstitial atom moves around
Typically has lower activation energy (happens more often)
What relationship governs the rate at which diffusion occurs?
Rate = c * e^(-Q/ RT)
Temp increases, rate increases
Q increases, rate decreases
What is activation energy with respect to diffusion?
The amount of energy required for diffusion to occur
How does temperature affect the diffusion process?
Higher temperature increases rate of diffusion
What is Fick’s 1st Law?
Fick’s first law of diffusion:
J = D * (dc/dx)
D = D_o * ( e^(-Q/RT) )
J: flux (atoms/cm^2/s)
D: diffusivity; diffusion coefficient (cm^2/s)
c: concentration (of one element in another) (atoms/cm^3)
dc/dx: concentration gradient (atoms/cm^3-cm)
Concentration gradient is the driving force for diffusion.
What is Ficks 2nd Law?
(c_s - c_x) / (c_s - c_o) = erf[ x / 2sqrt(Dt)]
c_s: constant concentration of the diffusing atoms at the surface of the material
c_x: g
