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DIFFUSION IN SOLIDS

DIFFUSION IN SOLIDS. http://web.mse.uiuc.edu/courses/mse280/. Gear from case-hardened steel (C diffusion). ISSUES TO ADDRESS. • How does diffusion occur?. • Why is it an important part of processing?. • How can the rate of diffusion be predicted for some simple cases?.

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DIFFUSION IN SOLIDS

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  1. DIFFUSION IN SOLIDS http://web.mse.uiuc.edu/courses/mse280/ Gear from case-hardened steel (C diffusion) ISSUES TO ADDRESS... • How does diffusion occur? • Why is it an important part of processing? • How can the rate of diffusion be predicted for some simple cases? • How does diffusion depend on structure and temperature?

  2. Diffusion • Mass transport by atomic motion • Mechanisms • Gases & Liquids – random (Brownian) motion • Solids – vacancy diffusion or interstitial diffusion • Cause (Driving force) • Concentration gradient

  3. x (mm) time (s) Diffusion: gases and liquids • Glass tube filled with water. • At time t = 0, add some drops of ink to one end of the tube. • Measure the diffusion distance, x, over some time. Possible to measure concentration

  4. Interdiffusion • Interdiffusion: in alloys, atoms tend to migrate from regions of large concentration. After some time Initially

  5. Selfdiffusion • Selfdiffusion: In an elemental solid, atoms also migrate. Label some atoms After some time This can be observed on metal surface under UHV conditions with scanning tunneling microscopy

  6. Vacancy diffusion or interstitial diffusion • applies to substitutional impurities • atoms exchange with vacancies • rate depends on (1) number of vacancies (2) activation energy to exchange. Concentration of Vacancies at temp T Vacancy The two processes have an activation energy (eV/atom). Interstitial

  7. Interstitial diffusion Concentration gradient More rapid than vacancy diffusion

  8. Processing using Diffusion • Case Hardening: -Diffuse carbon atoms into the host iron atoms at the surface. -Example of interstitial diffusion is a case hardened gear. • Result: The "Case" is -hard to deform: C atoms "lock" planes from shearing. -hard to crack: C atoms put the surface in compression.

  9. Processing using Diffusion • Doping Silicon with P for n-type semiconductors: • Process: 1. Deposit P rich layers on surface. 2. Heat it. 3. Result: Doped semiconductor regions.

  10. Modeling rate of diffusion: flux • Flux: • • Flux can be measured for: • - vacancies • - host (A) atoms • - impurity (B) atoms • Empirically determined: • – Make thin membrane of known surface area • – Impose concentration gradient • – Measure how fast atoms or molecules diffuse through the membrane A = Area of flow

  11. Steady-state Diffusion • Concentration Profile, C(x): [kg/m3] • Fick's First Law: D = m2/s D=diffusion coefficient • The steeper the concentration profile, the greater the flux!

  12. Steady-State Diffusion • Steady State: concentration profile not changing with time. Qm,r = Qm,l Qm,r Qm,l C(x) • Apply Fick's First Law: • since Qm,l = Qm,r then The slope, dC/dx, must be constant (i.e., doesn't vary with position)

  13. C1 C1 C2 x1 x2 C2 x Steady-State Diffusion Rate of diffusion independent of time

  14. 700 C Example: C Diffusion in steel plate • Steel plate at 7000C with geometry shown: Knowns: C1= 1.2 kg/m3 at 5mm (5 x 10–3 m) below surface. C2 = 0.8 kg/m3 at 10mm (1 x 10–2 m) below surface. D = 3 x10-11 m2/s at 700 C. In steady-state, how much carbon transfers from the rich to the deficient side?

  15. glove C1 paint remover skin C2 x1 x2 Example: Chemical Protection Clothing Methylene chloride is a common ingredient of paint removers. Besides being an irritant, it also may be absorbed through skin. When using, protective gloves should be worn. If butyl rubber gloves (0.04 cm thick) are used, what is the diffusive flux of methylene chloride through the glove? Data: D in butyl rubber: D = 110 x10-8 cm2/s surface concentrations: C1= 0.44 g/cm3 C2= 0.02 g/cm3 Diffusion distance: x2 – x1 = 0.04 cm

  16. Non-Steady-State Diffusion Concentration profile C(x) changes with time Qm,r Qm,l C(x) • To conserve matter: • Fick's First Law: • Governing Eqn.: • Fick's Second Law

  17. æ ö ç ÷ = D Do exp è ø EACT D = diffusion coefficient [m2/s] - Do R T = pre-exponential [m2/s] EACT = activation energy [J/mol or eV/atom] R = gas constant [8.314 J/mol-K] T = absolute temperature [K] Diffusion and Temperature • Diffusivity increases with T exponentially (so does Vacancy conc.).

  18. T(C) 1500 1000 600 300 10-8 D (m2/s) C in g-Fe C in a-Fe 10-14 Fe in a-Fe Fe in g-Fe Al in Al 10-20 1000K/T 0.5 1.0 1.5 • Experimental Data: Callister & Rethwisch 3e. from E.A. Brandes and G.B. Brook (Ed.) Smithells Metals Reference Book, 7th ed., Butterworth-Heinemann, Oxford, 1992.)

  19. 1000 1500 600 300 T(C) 10-8 D (m2/s) C in g-Fe C in a-Fe 10-14 Fe in a-Fe Fe in g-Fe Al in Al 10-20 1000 K/T 0.5 1.0 1.5 Example: Comparing Diffuse in Fe Is C in fcc Fe diffusing faster than C in bcc Fe? fcc-Fe: D0=2.3x10–5 (m2/s) EACT=1.53 eV/atom T = 900 C D=5.9x10–12(m2/s) bcc-Fe: D0=6.2x10–7(m2/s) EACT=0.83 eV/atom T = 900 C D=1.7x10–10(m2/s) • FCC Fe has both higher activation energy EACT and D0 (holes larger in FCC). BCC and FCC phase exist over limited range of T (at varying %C). Hence, at same T, BCC diffuses faster due to lower EACT. Cannot always have the phase that you want at the %C and T you want! which is why this is all important.

  20. Vacuum cases 1) Gas permeation through chamber’s walls The pressure gradient depends on the vacuum inside the chamber and is a steady state situation, after initial pumpdown 1° Fick’s law 2) Gas permeation from a solid located inside the chamber The initial concentration of gas si decreasing with time since the gas molecules are pumped away. (outgassing) 2° Fick’s law Relative concentration for a sheet of thickness d and different values of D/td2

  21. Material outgassing The role for UHV chambers Not all materials are good for UHV At RT, the flux stabilizes after few hours example After 20 h, the flux from untreated 3mm thick inox is 1 x 10-7 mbar L/(s cm2) So for surface area of 20000 cm2, the flux is 2 x 10-3 mbar L/s Pumping speed S = Q/p’ = 250 l/s p’ = 2x10-3/250 = 8 x10-6 mbar Inox machined and degreased after 24 h, flux 5 x 10-11 mbar L/(s cm2) So for surface area of 20000 cm2, the flux is 1 x 10-6 mbar L/s p’ = 1x10-6/250 = 4 x10-9 mbar Not included all evaporators, manipulators, etc In my lab 3 x 10-8 after two days

  22. Material outgassing Flux composition: H20 40% CO 20% CO2 12% So to remove H2O and decrease the outgassing flux from material you need to BAKE OUT The entire chamber and instruments for 48 h

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