Chapter 5: Diffusion

Transcription

1 Chapter 5: 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? Chapter 5-1

2 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids random (Brownian) motion Solids vacancy diffusion or interstitial diffusion Chapter 5 -

3 Interdiffusion: In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Diffusion After some time Adapted from Figs. 5.1 and 5., Callister & Rethwisch 8e. Chapter 5-3

4 Self-diffusion: In an elemental solid, atoms also migrate. Label some atoms C A D B Diffusion After some time C A B D Chapter 5-4

5 Vacancy Diffusion: Diffusion Mechanisms atoms exchange with vacancies applies to substitutional impurities atoms rate depends on: -- number of vacancies -- activation energy to exchange. increasing elapsed time Chapter 5-5

6 Diffusion Mechanisms Interstitial diffusion smaller atoms can diffuse between atoms. Adapted from Fig. 5.3(b), Callister & Rethwisch 8e. More rapid than vacancy diffusion Chapter 5-6

7 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. Adapted from chapter-opening photograph, Chapter 5, Callister & Rethwisch 8e. (Courtesy of Surface Division, Midland-Ross.) Result: The presence of C atoms makes iron (steel) harder. Chapter 5-7

8 Processing Using Diffusion Doping silicon with phosphorus for n-type semiconductors: Process: 0.5mm 1. Deposit P rich layers on surface. silicon. Heat it. 3. Result: Doped semiconductor regions. magnified image of a computer chip light regions: Si atoms silicon light regions: Al atoms Adapted from Figure 18.7, Callister & Rethwisch 8e. Chapter 5-8

9 Diffusion How do we quantify the amount or rate of diffusion? J Flux moles (or mass) diffusing mol or kg surface areatime cm s m s Measured empirically Make thin film (membrane) of known surface area Impose concentration gradient Measure how fast atoms or molecules diffuse through the membrane J M At l A dm dt M = mass diffused time J slope Chapter 5-9

10 Steady-State Diffusion Rate of diffusion independent of time Flux proportional to concentration gradient = dc dx C 1 C 1 Fick s first law of diffusion C C J D dc dx if linear dc dx x 1 x x D diffusion coefficient C x C x C x 1 1 Chapter 5-10

11 Example: Chemical Protective Clothing (CPC) 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 diffusive flux of methylene chloride through the glove? Data: diffusion coefficient in butyl rubber: D = 110x10-8 cm /s surface concentrations: C 1 = 0.44 g/cm 3 C = 0.0 g/cm 3 Chapter 5-11

12 Example (cont). Solution assuming linear conc. gradient C 1 paint remover glove x 1 x t b 6D C skin J Data: - D dc dx C D x D = 110x10-8 cm /s C 1 = 0.44 g/cm 3 C = 0.0 g/cm 3 x x 1 = 0.04 cm C x 1 1 J (110 x 10-8 cm (0.0 g/cm 0.44 g/cm /s) (0.04 cm) 3 3 ) 1.16 x 10-5 g cm s Chapter 5-1

13 Chapter 5-13

14 Chapter 5-14

15 Diffusion and Temperature Diffusion coefficient increases with increasing T. D D o exp Q d RT D D o Q d R T = diffusion coefficient [m /s] = pre-exponential [m /s] = activation energy [J/mol or ev/atom] = gas constant [8.314 J/mol-K] = absolute temperature [K] Chapter 5-15

16 Diffusion and Temperature D has exponential dependence on T T(C) D (m /s) Dinterstitial >> Dsubstitutional C in -Fe C in -Fe Al in Al Fe in -Fe Fe in -Fe K/T Adapted from Fig. 5.7, Callister & Rethwisch 8e. (Date for Fig. 5.7 taken from E.A. Brandes and G.B. Brook (Ed.) Smithells Metals Reference Book, 7th ed., Butterworth-Heinemann, Oxford, 199.) Chapter 5-16

17 Example: At 300ºC the diffusion coefficient and activation energy for Cu in Si are D(300ºC) = 7.8 x m /s Q d = 41.5 kj/mol What is the diffusion coefficient at 350ºC? D transform data ln D Temp = T 1/T lnd lnd lnd 0 lnd Q R 1 d 1 T D ln D 1 and Q R d lnd T T1 lnd 0 Q R d 1 T1 Chapter 5-17

18 Example (cont.) D D 1 exp Q R d 1 T 1 T 1 T 1 = = 573K T = = 63K D (7.8 x m /s) 41,500 J/mol exp J/mol - K 1 63 K K D = 15.7 x m /s Chapter 5-18

19 Chapter 5-19

20 Non-steady State Diffusion The concentration of diffusing species is a function of both time and position C = C(x,t) In this case Fick s Second Law is used Fick s Second Law C t D C x Chapter 5-0

21 VMSE: Student Companion Site Diffusion Computations & Data Plots Chapter 5-1

22 Non-steady State Diffusion Copper diffuses into a bar of aluminum. Surface conc., Cs of Cu atoms bar pre-existing conc., C o of copper atoms Cs Adapted from Fig. 5.5, Callister & Rethwisch 8e. B.C. at t = 0, C = C o for 0 x at t > 0, C = C S C = C o for x = 0 (constant surface conc.) for x = Chapter 5 -

23 Solution: C x,t C C C s o o 1 erf x Dt C(x,t) = Conc. at point x at time t erf (z) = error function z e y 0 dy erf(z) values are given in Table 5.1 C S C(x,t) C o Adapted from Fig. 5.5, Callister & Rethwisch 8e. Chapter 5-3

24 Non-steady State Diffusion Sample Problem: An FCC iron-carbon alloy initially containing 0.0 wt% C is carburized at an elevated temperature and in an atmosphere that gives a surface carbon concentration constant at 1.0 wt%. If after 49.5 h the concentration of carbon is 0.35 wt% at a position 4.0 mm below the surface, determine the temperature at which the treatment was carried out. C( x, t) C C C o Solution: use Eqn erf s o x Dt Chapter 5-4

25 Solution (cont.): C( x,t ) C C C s o o 1 erf x Dt t = 49.5 h x = 4 x 10-3 m C x = 0.35 wt% C s = 1.0 wt% C o = 0.0 wt% C( x, t) C C C s o o erf x Dt 1 erf( z) erf(z) = Chapter 5-5

26 Solution (cont.): We must now determine from Table 5.1 the value of z for which the error function is An interpolation is necessary as follows z erf(z) z z z Now solve for D z x Dt D x 4z t D x 4z t (4 x 10 (4)(0.93) 3 m) (49.5 h) 1h 3600 s.6 x m /s Chapter 5-6

27 Solution (cont.): To solve for the temperature at which D has the above value, we use a rearranged form of Equation (5.9a); from Table 5., for diffusion of C in FCC Fe D o =.3 x 10-5 m /s Q d = 148,000 J/mol T Qd R( lnd lnd) o T (8.314 J/mol - K)(ln.3x10 148,000 J/mol 5 11 m /s ln.6x10 m /s) T = 1300 K = 107ºC Chapter 5-7

28 Chapter 5-8

29 Chapter 5-9

30 Example: Chemical Protective Clothing (CPC) 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 breakthrough time (t b ), i.e., how long could the gloves be used before methylene chloride reaches the hand? Data diffusion coefficient in butyl rubber: D = 110 x10-8 cm /s Chapter 5-30

31 CPC Example (cont.) Solution assuming linear conc. gradient C 1 paint remover glove x 1 x C skin Breakthrough time = t b t b 6D x x1 D = 110x10-8 cm /s Equation from online CPC Case Study 5 at the Student Companion Site for Callister & Rethwisch 8e ( college/callister) 0.04 cm t b (0.04 cm) (6)(110 x 10-8 cm /s) 40 s 4 min Time required for breakthrough ca. 4 min Chapter 5-31

32 Chapter 5-3

33 Chapter 5-33

34 Summary Diffusion FASTER for... open crystal structures materials w/secondary bonding smaller diffusing atoms lower density materials Diffusion SLOWER for... close-packed structures materials w/covalent bonding larger diffusing atoms higher density materials Chapter 5-34