Diffusion Flashcards
Define diffusion
Diffusion - Mass transport by atomic motion.
Typically from an area of high concentration to an
area of low concentration.
Define Inter-diffusion and Self-diffusion
Inter-diffusion: In an alloy, atoms tend to migrate from regions
of high conc. to regions of low conc. (aka impurity diffusion)
Self-“diffusion”: In a pure solids, atoms also migrate. Much harder to see and no change in concentration profile.
Define vacancy diffusion
Vacancy diffusion (or substitutional diffusion) : • Atoms exchange with vacancies (point defects in metal structures) • The exchanging atom could be an original atom or an impurity (termed doping if the latter)
• The rate depends on:
- Number of vacancies in the material
- Activation energy to exchange (and therefore temperature)
- The frequency of hopping (influenced by temperature)
Define Interstitial diffusion
Interstitial diffusion:
•Smaller* atoms (C, H, O, N) can diffuse between the atoms that form the parent lattice structure (* based on atomic radius).
• More rapid than vacancy diffusion (more space to move in).
Discuss the processing of iron using diffusion
Processing iron using diffusion
High carbon iron fractures and crumbles when forged. Wrought iron, with nearly no carbon in it, is very malleable and ductile, but not very hard.
Low-carbon iron is heated in an atmosphere of C, N, or both, to encourage carbon or nitrogen diffusion into the surface of the iron.
A thin surface layer of higher carbon steel is formed, with the carbon content gradually decreasing the deeper we go from the surface.
The resulting product combines much of the toughness of a low-carbon steel core, with the hardness and wear resistance of the outer high-carbon steel.
• Case Hardening:
- Diffuse C or N atoms into the host iron atoms at the surface.
- Interstitial diffusion can be used in case hardening of self-drilling screws & nitride drill bits.
• Result: The presence of C and N atoms makes iron (screws) and titanium (drill bits) harder and temp resistant.
Discuss the doping of silicon using diffusion
Doping silicon (Si) with phosphorus for n-type semiconductors:
- Deposit phosphorus rich layers onto the surface of silicone
- Heat the material until diffusion has occurred
- Doped semiconductor regions are created within the silicone
How to quantify the rate of diffusion and how to empirically measure it.
J = Flux = Moles (or mass) diffusing / (Surface area x time) = kg/m2s
• Measured empirically
– Make thin film (membrane) of known surface area
– Impose concentration gradient (high conc. one side, zero on other)
– Measure how fast atoms/molecules diffuse through the membrane
State Fick’s first law of diffusion used to calculate Steady-State Diffusion
Fick’s first law of diffusion
J = -D(dC/dX)
J = Diffusion flux
D = Diffusion coefficient
C = Concentration
X = Distance
For now, assume the rate of diffusion is independent of time
Flux (J) proportional to concentration gradient = (dC/dX)
If the diffusion is linear = (C2 - C1) / (X2 - X1)
- Methylene chloride is a common ingredient of paint removers. Besides being an irritant, it also may be absorbed through skin. When using this paint remover, protective gloves should be worn.
- If butyl rubber gloves (0.04 cm thick) are used, what is the flux (J) of methylene chloride through the glove?
The diffusion coefficient in butyl rubber = D = 110x10-8cm2/s
The surface concentrations are:
C1 = 0.44 g/cm3
C2 = 0.02 g/cm3
J = -D(dC/dX) = -D(C2-C1 / X2 - X1)
Data:
D = 110 x 10-8 cm2/s
C1 = 0.44 g/cm3
C2 = 0.02 g/cm3
X2 - X1 = 0.04 cm
A plate of iron is exposed to a carburising (carbon rich) atmosphere on one side, and a de-carburising atmosphere on the other side at 700o C. During steady-state conditions, calculate the diffusion flux of carbon through the iron plate if the concerns of carbon at 5 and 10 mm beneath the carburising surface are 1.2 and 0.8 kg/m3, respectively. Assume a diffusion coefficient of 3 x10-11 m2/s at 700oC.
State the Equation utilised to calculate the diffusion coefficient
Calculate the diffusion coefficient for magnesium in aluminium at 550oC, given:
Rank the magnitudes of the diffusion coefficients from greatest to least for the following systems:
N in Fe at 700°C
Cr in Fe at 700°C
N in Fe at 900°C
Cr in Fe at 900°C
Justify this ranking given both Fe and Cr have BCC crystal structures, and the atomic radii for Fe, Cr, N are 0.124, 0.125, 0.065 nm, respectively.
Solution:
N in Fe at 900°C; DN(900)
N in Fe at 700°C; DN(700)
Cr in Fe at 900°C; DCr(900)
Cr in Fe at 700°C; DCr(700)
Nitrogen is an interstitial impurity in Fe (on the basis of its atomic radius), whereas Cr is a substitutional impurity. Since interstitial diffusion occurs more rapidly than substitutional impurity diffusion, DN > DCr.
As the magnitude of the diffusion coefficient increases with increasing temperature, D(900) > D(700).
At 300ºC the diffusion coefficient and activation energy for Cu in Si are:
D(300ºC) = 7.8 x 10-11 m<sup>2</sup>/s Qd = 41,500 J/mol
What is the diffusion coefficient at 350ºC?
D = D0 e(-Qd / RT) - Transforms - lnD = lnD0 (-Qd/R)(1/T)
Use Fick’s second law in the attached image and use it to solve the following:
During carburisation, a steel part is exposed to an atmosphere rich in methane (CH4) at an elevated temperature. The alloy initially has a uniform carbon concentration of 0.25 wt% and is to be treated at 950oC (1223 K). If the concentration of carbon at the surface is suddenly brought to and maintained at 1.20 wt%, how long will it take to achieve a carbon content of 0.8 wt% at a position of 0.5 mm below the surface?
The diffusion coefficient for carbon in iron = 1.6 x10-11 m2/s
Assume the steel part is semi-infinite
State 4 material properties in solids that facilitate faster diffusion and 4 that slow the diffusion process.
