Effect of divalent Ca2+ and Mg2+ substitutions on oxygen sensing properties of hot-spot based (Eu1-xCax)Ba2Cu3O7-δ and (Eu1-yMgy)Ba2Cu3O7-δ ceramic rods / Siti Azwani Yaacob

This thesis describes the effects of Ca2+ and Mg2+ substitution on oxygen sensing properties of hot spot-based Eul23 rods. (Eu1-xCax)Ba2Cu3O7-δ (x=0-0.5) and (Eu1-yMgy)Ba2Cu3O7-δ (y=0.05)ceramics were synthesized from oxide powders using the standard solid state method and fabricated into short rods. For unsubstituted x=0 rod the I-V behavior after formation of hot spot showed decreasing output current with increasing voltage under different pO2 concentration. However, for Ca�substituted rods, after appearance of a visible hot spot, a constant current plateau in the I-V curve was formed. The output current response of the rod in periodically changing pO2 between 20% and 100% showed improved stability and reproducibility for x=0.1, x-0.4 and x=0.5 compared to x=0.2 and x=0.3. Improved oxygen absorption and desorption time was observed for (Eu1-xCax)Ba2Cu3O7-δ (x=0.1, 0.4, 0.5) compared to the unsubstituted rod. Among the Ca-substituted samples, the x=0.4 rod produces oxygen absorption and desorption time of 64.5 s and 93.1 s, respectively. On the other hand, for Mg-substituted rods I-V behavior after formation of hot spot showed a negative slope. Faster absorption time of 3.0 s and desorption time of 6.9 s were observed for y=0.4 compared to other Mg-substituted rods. The improved output current stability, reproducibility and response time is suggested to be due to changes in oxygen activation energy and increased hole concentration as a result of Ca2+/Mg2+ substitutions. The Mg-substituted rods showed better performance compared to Ca-substituted rods possibly due to higher porosity and vacancy concentration. However, the difference in PTCR behavior for the same substitution level between Ca-substituted and Mg-substituted rods is suggested to be due to the differences in ionic size of Ca2+ andMg2+ which caused differences in reduction of oxygen activation energy. For the unsubstituted and Mg-substituted series, the relation between output current and pO2 shows a good agreement with the ideal case of oxygen excess material, derived from the mass action law.