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# HW5-new - ME311 MATERIALS ENGINEERING SPRING 2012...

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ME311 MATERIALS ENGINEERING SPRING 2012 HOMEWORK #5 DUE DATE: March 8, before class PROBLEM 1: You are given the task of purchasing a Microscope with the highest resolution possible. Among the instruments shown in the Table below, which microscope would you select? What is the best resolution for each radiation used? Assume the numerical aperture of the lens, NA=1. Microscope Radiation Used A X-rays B Ultraviolet C Green Light D Electrons E Red Light PROBLEM 2: The diffusivity of lithium in silicon is 10 -5 cm 2 /sec at 1376 K and 10 -6 cm 2 /sec at 968 K. What are the values for the enthalpy and the pre-exponential term for diffusion of lithium in silicon? Hint: Using the expression that relates the Diffusion coefficient with temperature, first remove the exponential terms to obtain an equation of a straight line. Subsequently, use a graphical form to answer the above questions. PROBLEM 3: Consider a diffusion couple setup between pure tungsten and a tungsten alloy containing 1 at% thorium. After several minutes of exposure at 2000 C, a transition zone of 0.01 cm thickness is established. What is the flux of thorium atoms at this time if the pre- exponential term (D o ) has a value of 1 cm 2 /s and the activation enthalpy for diffusion has a value of 120,000 cal/mole?

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