CHAPTER 6: DIFFUSION IN SOLIDS. Inter-diffusion. Simple Diffusion. Diffusion- Steady and Non-Steady State ISSUES TO ADDRESS...

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1 CHAPTER 6: DIFFUSION IN SOLIDS Diffusion- Steady and Non-Steady State ISSUES TO ADDRESS... Gear from case-hardened steel (C diffusion) Diffusion - Mass transport by atomic motion How does diffusion occur? Why is it an important part of processing? How can the rate of diffusion be predicted for some simple cases? Mechanisms Gases & Liquids random (Brownian) motion Solids vacancy diffusion or interstitial diffusion How does diffusion depend on structure and temperature? Simple Diffusion 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. Compare the results with theory. Interdiffusion: In alloys, atoms tend to migrate from regions of large concentration. Initially Inter-diffusion This is a diffusion couple. After some time Adapted from Figs , Callister 6e. 100% 0 Concentration Profiles 2 3 1

2 Self-diffusion Substitution-diffusion:vacancies and interstitials Self-diffusion: In an elemental solid, atoms also migrate. Label some atoms After some time C A D B applies to substitutional impurities atoms exchange with vacancies rate depends on (1) number of vacancies; (2) activation energy to exchange. Number (or concentration*) of Vacancies at T c i = n ΔE i N =e k T B k B T gives ev * see web handout for derivation. ΔE is an activation energy for a particular process (in J/mol, cal/mol, ev/atom). 4 Substitution-diffusion Vacancy Diffusion: applies to substitutional impurities atoms exchange with vacancies rate depends on (1) number of vacancies; (2) activation energy to exchange. Inter-diffusion across Interfaces Rate of substitutional diffusion depends on: - vacancy concentration - frequency of jumping. (Courtesy P.M. Anderson) increasing elapsed time Why should interstitial diffusion be faster than by vacancy mode of diffusion? 2

3 Diffusion Mechanisms Interstitial diffusion smaller atoms diffuse between atoms. 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. Fig. 6.0, Callister 6e. (courtesy of Surface Div., Midland-Ross.) More rapid than vacancy diffusion Result: The "Case" is -hard to deform: C atoms "lock" planes from shearing. -hard to crack: C atoms put the surface in compression. Doping Silicon with P for n-type semiconductors: Process: 1. Deposit P rich layers on surface. Processing using Diffusion Flux: Modeling rate of diffusion: flux Directional Quantity 2. Heat it. silicon 3. Result: Doped semiconductor regions. silicon Fig. 18.0, Callister 6e. 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 diffused mass M A = Area of flow time J slope 3

4 Steady-state Diffusion: J ~ gradient of c Steady-State Diffusion Concentration Profile, C(x): [kg/m 3 ] Concentration of Cu [kg/m 3 ] Cu flux Ni flux Adapted from Fig. 6.2(c) Concentration of Ni [kg/m 3 ] Steady State: concentration profile not changing with time. Position, x Fick's First Law: D is a constant! The steeper the concentration profile, the greater the flux! Apply Fick's First Law: If J x ) left = J x ) right, then J x = D dc dx dc = dc dx left dx right Result: the slope, dc/dx, must be constant (i.e., slope doesn't vary with position)! Steady-State Diffusion Rate of diffusion independent of time if linear dc dx C 2 C 2 dc J ~ dx Fick s first law of diffusion dc J = D dx x 1 x 2 D diffusion coefficient x ΔC C2 C = 1 Δx x2 x1 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 cm 2 /s surface concentrations: C C 2 = 0.02 g/cm 3 1 = 0.44 g/cm 3 Diffusion distance: x 2 x 1 = 0.04 cm glove paint remover x 1 x 2 2 l t b = 6D C 2 skin J x = -D C 2 - x 2 - x 1 = 1.16 x 10-5 g cm 2 s 4

5 Example: C Diffusion in steel plate Steel plate at C with geometry shown: Knowns: = 1.2 kg/m 3 at 5mm (5 x 10 3 m) below surface. C 2 = 0.8 kg/m 3 at 10mm (1 x 10 2 m) below surface. D = 3 x10-11 m 2 /s at 700 C. 700 C Adapted from Fig. 5.4, Callister 6e. Example: Diffusion of radioactive atoms Surface of Ni plate at C contains 50% Ni 63 (radioactive) and 50% Ni (non-radioactive). 4 microns below surface Ni 63 /Ni = 48:52 Lattice constant of Ni at 1000 C is nm. Experiment shows that self-diffusion of Ni is 1.6 x 10-9 cm 2 /sec What is the flux of Ni 63 atoms through a plane 2 µm below surface? = (4Ni /cell)(0.5ni63 /Ni) (0.36x10 9 m) 3 /cell = 42.87x10 27 Ni 63 /m 3 C 2 = (4Ni /cell)(0.48ni63 /Ni) (0.36x10 9 m) 3 /cell = 41.15x10 27 Ni 63 /m 3 Q: In steady-state, how much carbon transfers from the rich to the deficient side? = (1.6x10 13 m 2 /sec) ( )x1027 Ni 63 /m 3 (4 0)x10 6 m = +0.69x10 20 Ni 63 /m 2 s How many Ni 63 atoms/second through cell? J (0.36nm) 2 = 9 Ni 63 /s Where can we use Fick s Law? Non-Steady-State Diffusion Fick's law is commonly used to model transport processes in foods, clothing, biopolymers, pharmaceuticals, porous soils, semiconductor doping process, etc. Concentration profile, C(x), changes w/ time. To conserve matter: Fick's First Law: Example The total membrane surface area in the lungs (alveoli) may be on the order of 100 square meters and have a thickness of less than a millionth of a meter, so it is a very effective gas-exchange interface. CO 2 in air has D~16 mm 2 /s, and, in water, D~ mm 2 /s Governing Eqn.: 14 5

6 Non-Steady-State Diffusion: another look Concentration profile, C(x), changes w/ time. Rate of accumulation C(x) Non-Steady-State Diffusion: C = c(x,t) concentration of diffusing species is a function of both time and position Fick's Second "Law" Copper diffuses into a bar of aluminum. c D c 2 t x 2 C t dx = J x J x+dx Using Fick s Law: C t C t dx = J x (J x + J x x dx) = J x x dx = J x x = C D x x Fick s 2nd Law Cs B.C. at t = 0, C = C o for 0 x at t > 0, C = C S for x = 0 (fixed surface conc.) C = C o for x = If D is constant: Fick's Second "Law" c = D c D c 2 t x x x 2 Adapted from Fig. 6.5, Callister & Rethwisch 3e. 14 Non-Steady-State Diffusion Cu diffuses into a bar of Al. C S Example: Non-Steady-State Diffusion FCC iron-carbon alloy initially containing 0.20 wt% C is carburized at an elevated temperature and in an atmosphere that gives a surface C content at 1.0 wt%. Fick's Second "Law": c D c 2 t x 2 C(x,t) C o If after 49.5 h the concentration of carbon is 0.35 wt% at a position 4.0 mm below the surface, what temperature was treatment done? Solution C( x, t) Co x = = 1 erf = 1 erf( z) Cs Co Dt erf(z) = Solution: "error function Values calibrated in Table 6.1 erf (z) = 2 2 e y dy 0 π 15 z Using Table 6.1 find z where erf(z) = Use interpolation. z = So, z = x 2 x Now solve for D z = D = 2 Dt 2 4z t x 2 (4 x 10 3 m) 2 1 h D = = 4z 2 t (4)(0.93) 2 (49.5 h) 3600 s = 2.6 x m 2 /s z erf(z) z

7 Solution (cont.): To solve for the temperature at which D has the above value, we use a rearranged form of Equation (6.9a); D=D 0 exp(-q d /RT) From Table 6.2, for diffusion of C in FCC Fe Q T = d R( lndo lnd) D o = 2.3 x 10-5 m 2 /s Q d = 148,000 J/mol D = 2.6 x m 2 /s 148,000 J/mol T = (8.314 J/mol-K)(ln 2.3x10 5 m 2 /s ln 2.6x10 11 m 2 /s) T = 1300 K = 1027 C Copper diffuses into a bar of aluminum. 10 hours processed at 600 C gives desired C(x). How many hours needed to get the same C(x) at 500 C? Key point 1: C(x,t 500C ) = C(x,t 600C ). Key point 2: Both cases have the same C o and C s. Result: Dt should be held constant. Answer: Example: Processing C(x,t) C o x = 1 erf C s C o 2 Dt Note D(T) are T dependent! Values of D are provided. 16 Diffusion Analysis The experiment: we recorded combinations of t and x that kept C constant. Data from Diffusion Analysis C(x i,t i ) C o C s C o x = 1 erf i 2 Dt i = (constant here) Diffusion depth given by: Experimental result: x ~ t 0.58 Theory predicts x ~ t 0.50 from Close agreement. 17 7

8 Diffusion and Temperature Diffusivity increases with T exponentially (so does Vacancy conc.). D = D o exp Q d RT D = diffusion coefficient [m 2 /s] D o Q d R T = pre-exponential [m 2 /s] (see Table 6.2) = activation energy [J/mol or ev/atom] = gas constant [8.314 J/mol-K] = absolute temperature [K] Experimental Data: Adapted from Fig. 6.7, D (m 2 /s) Diffusion and Temperature C in γ-fe Fe in γ-fe Fe in α-fe 600 C in α-fe Al in Al T( C) Dinterstitial >> Dsubstitutional C in α-fe Cu in Cu C in γ-fe Al in Al Fe in α-fe Fe in γ-fe Zn in Cu 1000 K/T Note: ln D = ln D 0 Q d R 1 T log D = log D 0 Q d 1 2.3R T Adapted from Fig. 6.7, Callister & Rethwisch 3e. (Data for Fig. 6.7 from E.A. Brandes and G.B. Brook (Ed.) Smithells Metals Reference Book, 7th ed., Butterworth-Heinemann, Oxford, 1992.) Example: Comparing Diffuse in Fe Is C in fcc Fe diffusing faster than C in bcc Fe? (Table 6.2) fcc-fe: D 0 =2.3x10 5 (m 2 /2) Q=1.53 ev/atom T = 900 C D=5.9x10 12 (m 2 /2) bcc-fe: D 0 =6.2x10 7 (m 2 /2) Q=0.83 ev/atom T = 900 C D=1.7x10 10 (m 2 /2) D (m 2 /s) 10-8 T( C) C in γ-fe Fe in γ-fe Fe in α-fe C in α-fe Al in Al FCC Fe has both higher activation energy Q and D 0 (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 Q. Cannot always have the phase that you want at the %C and T you want! which is why this is all important K/T Connecting Holes, Diffusion, and Stress z-axis Red octahedral fcc Green + Red octahedral bcc FCC represented as a BCT cell, with relevant holes. 8

9 concentration Other Types of Diffusion (Beside Atomic) Flux is general concept: e.g. charges, phonons, Charge Flux jq = 1 dq A dt = 1 d(ne) A dt Defining conductivity σ j = σ dv (a material property) dx Ohm s Law j q = I A = 1 V A R = σ V l e = electric chg. N = net # e- cross A Solution: Fick s 2nd Law V x V s x = erf V 0 V s 2 σt Inter-diffusion (diffusion couples) Concentration of Cu Concentration [kg/m 3 ] of Cu [kg/m 3 ] C 2 Cu flux Cu flux C 0 interface Ni flux Ni flux Position, x Position, x Concentration Concentration of Ni [kg/m 3 ] of Ni [kg/m 3 ] Heat Flux (by phonons) Defining thermal conductivity κ (a material property) Or w/ Thermal Diffusivity: Q = 1 dh = 1 d(nε) A dt A dt h = κ c V ρ Q = κ dt dx Q = c V ρt N = # of phonons with avg. energy ε Solution: Fick s 2nd Law T x T s x = erf T 0 T s 2 ht Assuming D A = D B : For C 2 x C x 1 C 0 x = erf C 0 2 Dt C 0 = ( +C 2) /2 Curve is symmetric about C 0, erf ( z) = erf ( z) Kirkendall Effect: What is D A > D B? Kirkendall studied Mo markers in Cu-brass (i.e., fcc Cu 70 Zn 30 ). Symmetry is lost: Zn atoms move more readily in one direction (to the right) than Cu atoms move in the other (to the left). Cu Zn Cu 70 Zn 30 Cu Mo wire markers t = 0 Diffusion in Compounds: Ionic Conductors Unlike diffusion in metals, diffusion in compounds involves second-neighbor migration. Since the activation energies are high, the D s are low unless vacancies are present from non-stoichiometric ratios of atoms. e.g., NiO There are Schottky defects O Ni O Ni O Ni O Ni O Ni O Ni 2+ O 2 Ni O Ni 2+ O 2 Ni O 2 + O When diffusion is asymmetric, interface moves away markers, i.e., there is a net flow of atoms to the right past the markers. Analyzing movement of markers determines D Zn and D Cu. Kirkendall effect driven by vacancies, effective for T > 0.5 T melt. After diffusion Ni Ni 2+ O Ni O Ni O Ni O Ni O Ni O The two vacancies cannot accept neighbors because they have wrong charge, and ion diffusion needs 2nd neighbors with high barriers (activation energies). 9

10 Diffusion in Compounds: Ionic Conductors D s in an ionic compound are seldom comparable because of size, change and/or structural differences. Two sources of conduction: ion diffusion and via e - hopping from ions of variable valency, e.g., Fe 2+ to Fe 3+, in applied electric field. e.g., ionic In NaCl at 1000 K, D Na+ ~ 5D Cl,whereas at 825 K D Na+ ~ 50D Cl! This is primarily due to size r Na+ = 1 A vs r Cl =1.8 A. e.g., oxides In uranium oxide, U 4+( O 2 ) 2, at 1000 K (extrapolated), D O ~ 10 7 D U. This is mostly due to charge, i.e. more energy to activate 4+ U ion. Also, UO is not stoichiometric, having U 3+ ions to give UO 2-x, so that the anion vacancies significantly increase O 2- mobility. e.g., solid-solutions of oxides (leads to defects, e.g., vacancies) If Fe 1-x O (x=2.5-4% at 1500 K, 3Fe 2+ -> 2Fe 3+ + vac.) is dissolved in MgO under reducing conditions, then Mg 2+ diffusion increases. If MgF 2 is dissolved in LiF (2Li + -> Mg 2+ + vac.), then Li + diffusion increases. All due to additional vacancies. Ionic Conduction: related to fuel cells Molten salts and aqueous electrolytes conduct charge when placed in electric field, +q and q move in opposite directions. The same occurs in solids although at much slower rate. Each ion has charge of Ze (e = 1.6 x amp*sec), so ion movement induces ionic conduction Conductivity σ = nµze is related to mobility, µ, which is related to D via the Einstein equations: µ = ZeD / k B T Hence log 10 σ ionic ~ ln nz 2 e 2 k B T σ ionic = nz 2 e 2 D o Q 2.3RT k B T D = nz 2 e 2 k B T D oe Q / RT So, electrical conduction can be used determine diffusion data in ionic solids. e.g., What conductivity results by Ca 2+ diffusion in CaO at 2000 K? CaO has NaCl structure with a= 4.81 A, with D(2000 K)~10-14 m 2 /s, and Z=2. n Ca 2+ = 4 cell cell (4.81x10 10 m) = x1028 / m 3 σ = nz 2 e 2 k B T D ~ 1.3x10 5 ohm cm Example: solid-oxide fuel cell (SOFC) Ceramic Compounds: Al 2 O 3 SOFC is made up of four layers, three of which are ceramics (hence the name). A single cell consisting of these four layers stacked together is only a few mm thick. Need to stack many, many together to have larger DC current. Overall: H 2 + 1/2O 2 H 2 O Holes for diffusion H 2 2H + +2e O 2 + 2H + + 2e H 2 O Image: S. Haile s SOFC (2004 best): Thin-film of Sm-doped Ceria electrolyte (CeO 2, i.e. Sm x Ce 1-x O 2-x/2 ) and BSCF cathode (Perovskite Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-d ) show high power densities over 1 W/cm 2 at 600 C with humidified H 2 as the fuel and air at the cathode. Unit cell defined by Al ions: 2 Al + 3 O 10

11 Summary: Structure and Diffusion Diffusion FASTER for... open crystal structures lower melting T materials materials w/secondary bonding smaller diffusing atoms cations lower density materials Diffusion SLOWER for... close-packed structures higher melting T materials materials w/covalent bonding larger diffusing atoms anions higher density materials 11