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Problem Set IV
Ceramics Materials Chemistry and Synthesis
1) The oxidation rate of cobolt to CoO at 1148°C is proportional to the partial
pressure of oxygen raised to the one-fourth power.
a) Explain this effect quantitatively.
b) From your answer in Part a), what is the diffusing species?
2) Consider oxidation of pure, solid Mg by O2 gas. Markers placed on the
unoxidized Mg surface prior to exposure to oxygen were found on the MgO/O2
surface after oxidation. At the temperatures investigated in the experiment, the
oxide thickness was found to be strictly parabolic. Assuming the oxidation rate
constant is proportional to ionic diffusivity via vacancies:
a) What are the diffusing species, and in what direction (towards or away from
the Mg/MgO interface) do they diffuse?
b) Assuming an activation energy for the ionic diffusivity is 370,000 J/mole,
what is the ratio of oxide thicknesses for a test temperature of 450°C versus
500°C? (R = 8.314 J/mole °K.)
3) The oxidation rate of zinc in oxygen at 1 atm and 390°C increases by a factor of
10 when (0.4 wt. pct.) Li is added to pure zinc and decreases by a factor of 50
when (1 wt. pct.) Al is added to pure zinc. Assume oxidation of Zn is due to
movement of zinc as an interstitial in the oxide at these dopant levels and
lower.
a) Explain the doping behavior semi-quantitatively based on a defect model
for ZnO and a defect equilibrium for each dopant at these levels as Li2O and
Al2O3, involving only ionic species.
b) For the addition of Al at this level to the pure zinc, what is an alternative
explanation involving the creation of electrons?
c) It is found that, with increasing aluminum levels beyond 1 % Al, a minimum
is reached in the diffusivity of ionic zinc with additions of Al, thereafter,
rising with concentration of Al. How would you explain this anomaly
qualitatively?
University of Cincinnati MTEN6053 S19 © D. Kundrat