A Sample Test 1.TiO2has been developed as an oxygen sensor material based on the its electronic conductivity dependence on oxygen pressure. The electronic conductivity is proportional to the electron concentrations in the crystal. Experimental measurements showed that in oxygen-deficient Titania (TiO2-x) in which the defect concentrations are controlled by the reduction reaction, the electronic conductivity is proportional to minus fifth power of oxygen pressure, i.e., nelectron concentration()∝pO2−1 5There are two possible reduction mechanisms in TiO2-x: (1) the oxygen deficiency is accommodated by oxygen vacancies; and (2) the oxygen deficiency is caused by titanium interstitials. (a)Write the defect reactions for the two possible reduction mechanisms. (b)Based on the above dependence of electron concentration on oxygen pressure, determine which of the two is the dominant mechanism? 2. Discuss the three regimes of grain boundary diffusion, A, B and C, as classified by Harrison: (a)
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