Diffusion Flashcards
Define diffusion
Movement of matter down an activity gradient (due to a driving force)
How is atom concentration calculated?
Ca = Na/V Concentration = number of atoms/volume
Describe uphill and downhill diffusion, when they occur and what they mean
Downhill = when lowest G is when AB is completely mixed, they will diffuse across interface until completely mixed Uphill = when lowest G is when A+B are separate, atoms will not move across interface and will move along activity gradient but away from concentration gradient
Describe carburisation
Carburisation - adding carbon to a material with unlimited carbon supply at surface, as time inc more carbon diffused into material + effects deeper layers
Diffusion stops when carbon level in centre of material = carbon in air
Describe inter-diffusion
Diffusant moves from one sample to another, unlimited supply and concentration not fixed at interface as atoms diffuse in both directions
As time inc diffusion effects deeper layers of the material
Describe thin film diffusion
Finite amount of diffusant
Means that when t inc the amount of diffusant levels out across material until all same level
Describe homogenisation
Diffusant fluctuations across material, as time increases diffusants mix into material so concentration decreases
X = root (Dt)
X = distance, D = diffusion constant, t=time
What is Ficks 1st law?
J = -D. δc/δx J = diffusion flux (amount of diffusion), D = diffusion coefficient (mat property), c = concentration, x = distance
Derive ficks 1st law
Interface separating 2 materials, flux across surface = atoms crossing interface/Sarea.time, flux is proportional to concentration, flux is inversely proportional to distance atoms travel, means flux = kδc/δx with k = diffusion coefficient
What is the diffusion coefficient and what’s it effected by?
Measure of how fast an atom diffused through a material
Affected by heat, atom size, valence, crystal structure and diffusion mechanism used
How is diffusivity Coefficient measured?
Take a metal specimen and place a radioactive isotope of the metal around it, place into a furnace at a set temp for a set time, quench material and remove isotope film, use detectors to measure concentration of isotope on sample, plot graph using ficks 2 and determine D for that temp
What is ficks 2nd law?
δC/δt = D. δ2C/δx2 C = concentration, t = time, x = distance, D = diffusivity coefficient
How is diffusion’s relationship with temp measured?
Repeat diffusivity experiment at multiple temperatures, then plot ln D vs 1/T to get a linear relationship
What is diffusivitys relationship with temp?
D = D0.e-(Q/RT) D = diffusivity, D0 = pre-exponential factor, Q = activation energy, R = universal gas constant l, T = absolute temp
How is diffusion rate calculated?
J = atoms crossing/surface area.time
How does crystal structure, bonding characteristics and vacancy concentration affect diffusivity?
1- open structures (BCC) have lower activation energies = easier to move through
2- stronger bonds take greater energy to break = higher activation energies
3- more vacancy’s = less bonds to break = less activation energy = higher diffusion
What affects diffusivity more, activation energy or the preexponetial factor?
Activation energy, if this is high then D = D0 meaning high diffusion rates
What are the four diffusion mechanisms?
Interstitial
Vacancy
Grain boundary
Pipe
Describe interstitial diffusion
Only a fraction of interstitials occupied (BCC least), diffusant moves between interstitials but random movement into any of the 6 neighbouring positions (3D lattice)
D = 1/6 . a2 . r
D = diffusion, a = diffusant atoms, r = jump frequency
What dictates the jump frequency in interstitial diffusion?
How many atomic sites an atom can move to
Frequency of vibrations
Activation energy (as only a fraction of atoms can jump each time)
Describe vacancy diffusion
Atom moves into vacancy, must be a vacancy near atom for it to move and atom leaves vacancy when it moves = fairly likely for atom to move back as not many vacancies in material
D = 1/6 . a2 . f . r
D = diffusion rate, a = number of atoms, f = jump back rate, r = jump frequency
Compare interstitial and vacancy diffusion
Interstitial much quicker diffusion because lower activation energy (interstitial = migration energy, vacancy = migration + formation energy)
In both bigger atoms diffuse slower (as take more migration energy)
Bonding strength also affects activation energy (stronger = slower diffusion)
Define migration energy
Energy taken for atom to move down pathway
Bigger atoms = larger migration energy
Larger pathway = smaller migration energy
Describe grain boundary and pipe diffusion
Grain boundary - sets up easy path to move down as resembles an amorphous structure (less dense packed) = lower energy required
Pipe - rare (so usually ignored) but cause a ‘pipe’ pathway down lattice, lowers activation energy as already strained lattice = quick diffusion
