Question

1. The maximum weight percentage of carbon atoms (which are very small atoms) that can be contained in spaces between the iron atoms in a crystal of face-centered cubic iron at 1333 deg F (723 degC) is 0.8%C.

(a) To what approximate atomic percentage does this correspond?

(b) On the average, how many unit cells of face-centered cubic iron would have to be examined before encountering a carbon atom?

2. (a) Briefly explain the concept of driving force.

(b) What is the driving force for steady-state diffusion.

3. (a) Compare interstitial and vacancy atomic mechanisms for diffusion.

(b) Cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion.

4. Consider the purification of hydrogen gas by diffusion through a palladium sheet. Compute the number of kilograms of hydrogen that pass per hour through a 6 [mm] thick sheet of palladium having an area of 0.25 [m2] at 600 °C. Assume a diffusion coefficient of 1.7 x 10-8 [m2/s], that the respective concentrations at the high- and low-pressure sides of the plate are 2.0 and 0.4 [kg] of hydrogen per cubic meter of palladium, and that steady-state conditions have been attained.

PLEASE SOLVE ALL QUESTIONS& EXPLAIN IN DETAIL

Answer 1

1)Mass of Carbon = 12 u

Mass of Iron = 55.845 u

The crystal has 0.8% C by mass and 99.2% Fe by mass.

Let the mass of crystal be 'm'. Thus mass of C in this is 0.008m.

thus 1 atom of C will be contained in 12/0.008 u = 1500u.

this will have 1488u of Fe = 26.64 atoms.

thus there will be 1 C atoms for 26.64 Fe atoms. this corresponds to an atomic percentage of 3.75%.

as one fcc unit cell contains 4 atoms, we need to examine ~7 unit cells before encountering a C atom.