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ENMA300_HW10 - x through the same Al 2 O 3 crystal at...

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ENMA300: Homework #10 Date Posted: Tue 11/23/2010 Time Due: Tue 11/30/2010 3 PM. 1. Under what conditions can Fick’s first law be used to solve diffusion problems? 2. Silica can be either a crystalline solid or a glass at room temperature. Is diffusion faster in glassy or crystalline silica? Why? 3. Ge is covalently bonded while Cu possess metallic bonds. Self-diffusion in each takes places via vacancy-exchange mechanism. For comparable temperatures, which would have a larger diffusion coefficient? Why? 4. Compare the diffusion coefficient of methane in rubber [D=3*10 -6 m 2 /s] at 293 K with the diffusion coefficient of Cu in Ag [D 0 = 1.2*10 -4 m 2 /s; Q diff = 193 kJ/mol] at the same temperature. Is this result expected? To what do you attribute the large difference in diffusion rates? 5. Suppose it takes 3 hours for an Al atom to diffuse a distance x through an Al 2 O 3 crystal at T=1000°C. [D 0 = 2.8*10 -3 m 2 /s; Q diff = 477 kJ/mol] a. Estimate the time required for an Al atom to diffuse a distance of 9
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Unformatted text preview: x through the same Al 2 O 3 crystal at T=1000°C. b. Calculate the temperature at which the diffusion rate of Al through an Al 2 O 3 crystal will be 10 times that of T=1000°C. 6. a. Describe the mechanisms of degradation of polymeric materials. b. Why would you not store benzene in polystyrene containers? Suggest a better alternative (describe pros and cons of the suggested material). 7. Explain ‘cathodic protection’. How does it work? What are the material requirements (composition and shape) of the sacrificial anode? 8. At one instance of time, there is a constant gradient in concentration of Cu in an Al crystal: 0.19%at Cu at the surface and 0.18%at Cu at a depth of 1.2mm below the surface. The diffusion coefficient of Cu in Al at the temperature of the system is D= 4*10-14 m 2 /s. The lattice parameter of FCC Al is 4.049Å. What is the flux of Cu atoms, and in which direction do they flow?...
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