Chapter 5: Diffusion Flashcards
Steady state diffusion
The thermally activated kinetic movement of atoms down a concentration gradient.
Steady state diffusion influences (4)
Temperature, crystal structure, diffusion mechanism, activation energy
What affects steady state diffusion (3)
Chemical potential gradient, thermal energy, atomic vibrations
Steady state diffusion mechanisms (2)
substitutional diffusion and interstitial diffusion
Substitutional diffusion
Thermally activated vacancy-atom exchange where the atoms pass through a high-energy position since the nearby atoms must spread apart.
Relationship between substitutional diffusion and temperature
Diffusion rate and diffusion coefficient increases exponentially with temperature. Activation energy depends on the energy required to jump and the number of vacancies (which depends exponentially on the formation enthalpy of a vacancy)
Qdiffusion = Qjump + Qvacancies
Interstitial diffusion
Atoms diffuse through empty space within the crystal. Activation energy is lower and the reaction is faster (particularly at low T).
Fick’s first law
Describes steady state diffusion by the flux (J) of atoms moving through the concentration gradient.
Factors affecting diffusion (3)
- Diffusion mechanism
- Arrhenius equation (temperature)
- Diffusivity
Diffusivity
Rate of diffusion, described by the diffusion coefficient.
Plotting Diffusivity and Temperature
Can be plotted as D vs. 1/T to get the activation energy (slope) and Do (y intercept).
Relationship between Diffusivity and activation energy
The higher the activation energy, the slower the diffusion at lower temperatures
Diffusivity of materials
Larger as you exit the material
Dsurface > Dgrain boundary > Dbulk
Fick’s second law
Flux and concentration changes with time.
