Diffusion

Question 1

Define diffusion

Answer 1

Movement of matter down an activity gradient (due to a driving force)

Question 2

How is atom concentration calculated?

Answer 2

Ca = Na/V 
Concentration = number of atoms/volume

Question 3

Describe uphill and downhill diffusion, when they occur and what they mean

Answer 3

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

Question 4

Describe carburisation

Answer 4

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

Question 5

Describe inter-diffusion

Answer 5

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

Question 6

Describe thin film diffusion

Answer 6

Finite amount of diffusant

Means that when t inc the amount of diffusant levels out across material until all same level

Question 7

Describe homogenisation

Answer 7

Diffusant fluctuations across material, as time increases diffusants mix into material so concentration decreases
X = root (Dt)
X = distance, D = diffusion constant, t=time

Question 8

What is Ficks 1st law?

Answer 8

J = -D. δc/δx
J = diffusion flux (amount of diffusion), D = diffusion coefficient (mat property), c = concentration, x = distance

Question 9

Derive ficks 1st law

Answer 9

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

Question 10

What is the diffusion coefficient and what’s it effected by?

Answer 10

Measure of how fast an atom diffused through a material

Affected by heat, atom size, valence, crystal structure and diffusion mechanism used

Question 11

How is diffusivity Coefficient measured?

Answer 11

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

Question 12

What is ficks 2nd law?

Answer 12

δC/δt = D. δ2C/δx2
C = concentration, t = time, x = distance, D = diffusivity coefficient

Question 13

How is diffusion’s relationship with temp measured?

Answer 13

Repeat diffusivity experiment at multiple temperatures, then plot ln D vs 1/T to get a linear relationship

Question 14

What is diffusivitys relationship with temp?

Answer 14

D = D0.e-(Q/RT) 
D = diffusivity, D0 = pre-exponential factor, Q = activation energy, R = universal gas constant l, T = absolute temp

Question 15

How is diffusion rate calculated?

Answer 15

J = atoms crossing/surface area.time

Question 16

How does crystal structure, bonding characteristics and vacancy concentration affect diffusivity?

Answer 16

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

Question 17

What affects diffusivity more, activation energy or the preexponetial factor?

Answer 17

Activation energy, if this is high then D = D0 meaning high diffusion rates

Question 18

What are the four diffusion mechanisms?

Answer 18

Interstitial
Vacancy
Grain boundary
Pipe

Question 19

Describe interstitial diffusion

Answer 19

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

Question 20

What dictates the jump frequency in interstitial diffusion?

Answer 20

How many atomic sites an atom can move to
Frequency of vibrations
Activation energy (as only a fraction of atoms can jump each time)

Question 21

Describe vacancy diffusion

Answer 21

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

Question 22

Compare interstitial and vacancy diffusion

Answer 22

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)

Question 23

Define migration energy

Answer 23

Energy taken for atom to move down pathway
Bigger atoms = larger migration energy
Larger pathway = smaller migration energy

Question 24

Describe grain boundary and pipe diffusion

Answer 24

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