Chapter 5 : Thermally Activated Process and Diffusion in Solids Flashcards
1. rate process in solids, concept of activation energy 2. describe the effect of temperature on reaction rates based off the Arrhenius rate equation 3. two main mechanisms of diffusion 4. steady and non-steady diffusion and Fick's laws 5. industrial applications of diffusion
What is diffusion?
mechanism in which matter is transported through matter.
What are the two mechanisms of diffusion?
- vacancy/substitution mechanism
2. interstitial mechanism
What is the relationship of temperature to diffusion?
- the temp will rise, do to more vacancies. The more vacancies there are, the more thermal energy is available.
- Diffusion rate is higher at higher temp (increases exponentially)
- Higher melting point = higher activation energy.
(due to the stronger bonds resisting the change of heart/energy).
What is self diffusion?
when an atom gains enough energy and moves into a vacant spot (spot must be adjacent). Always substitutional.
Describe Interstitial diffusion.
when an atom from one interstitial site moves into another without permanently displacing atoms in the process. faster than substitutional (easier for smaller atoms to move than larger atoms)
What is activation energy?
additional energy required above the average energy of the atoms
Steady State diffusion is?
when the conditions in diffusion do not change with time. The concentration remains the same, as concentration is increased on one side, it decreases on the other (concentration input flows from higher to lower) Ficks first law
Frick’s first law of diffusion states?
Net flow of atoms is equal to the diffusivity times the gradient. J=-D*dc/dx, where J = flux, D = diffusivity constant, dc/dx = concentration gradient.
steady state diffusion
What are the units for Frick’s first law?
atoms/(m^2*s) = (m^2/s)(atoms/m^4)
Diffusivity depends on what 5 variables?
- type of diffusion mechanism
- temperature that diffusion takes place
- type of crystal structure of the solvent
- type of crystal imperfections present
- the concentration of diffusing species
Fick’s first law of diffusion discuses steady state flow. What does Fick’s second law of diffusion describe?
Non steady state diffusion.
What is Fick’s second, easier equation for this second law?
(Cs-Cx)/(Cs-Co)=erf(x/2sqrt(Dt)) Cs = surface concentration Cx = concentration at depth x = depth Co = initial concentration D = diffusivity constant erf = error function (will need your book/table to calculate)
What is gas carburizing? and the describe the process.
the diffusion of carbon via gas into a metal.
metals are put under extreme heat and hydrocarbon gases release carbon into the metal. the carbon diffuses into the metal.
What factors affect the diffusion rate?
Type of crystal imperfections present Concentration of the diffusion species (concentration gradient) Type of crystal structure Type of diffusion mechanism Temperature of diffusion
