| Summary: | 碩士 === 國立臺灣科技大學 === 材料科技研究所 === 94 === Co-doping effect of various ionic radii of divalent (Sr2+, Ca2+, Mg2+) and trivalent (Y3+) cations on ionic conductivity of zirconia was investigated using x-ray diffractiometry, scanning electron microscopy, transmission electron microscopy, raman spectroscopy and ac impedance analysis in this work.
For understanding the principles of enhancing the ionic conductivity of zirconia, (8YSZ)X-(12CSZ)1-X was investigated in the first topic. Experimental results show the ionic conductivity of (8YSZ)X-(12CSZ)1-X decreases with an increase of 12CSZ content. The (8YSZ)X-(12CSZ)1-X are all cubic phase calculated from x-ray diffraction patterns, but a 480 cm-1 extra peak was found from raman scattering patterns in (8YSZ)0.5-(12CSZ)0.5. It seems that another unusual phase was formed; for this reason, the microstructral feature of (8YSZ)0.5-(12CSZ)0.5 was carried out using transmission electron microscopy. The t’ phase and a peculiar monoclinic phase (m’) was observed, and the defect association or lattice distortion would be produced in (8YSZ)X-(12CSZ)1-X due to too many oxygen vacancies, which is the possible reason that the ionic conductivity (8YSZ)X-(12CSZ)1-X decreases with an increase of 12CSZ content.
Y2O3 doping concentration in zirconia of 8mol% exhibits highest ionic conductivity, and the ionic conductivity of Y2O3 doped zirconia decreased when the composition of Y2O3 higher than 8 mol% because of the oxygen vacancy clustering. In order to decrease and restrain the average binding energy and oxygen vacancy clustering in zirconia respectively for enhancing ionic conductivity, the divalent cations (Sr2+, Ca2+, Mg2+) and trivalent (Y3+) cations doped zirconia which possess the lowest lattice binding energy and the radius of doped cations close to Zr4+ was devised. Besides, the amount of doped cations of co-doping system must be fixed at 8 mol%, and the amount of oxygen vacancy was controlled around 8-9 mol% in YSZ matrix. The results demonstrate that the specimens of Zr0.92Y0.16-XMXO2.08-0.5X are of cubic structure calculated from x-ray diffraction patterns. It is found the best conductivity of Zr0.920Y0.155Mg0.005O2.08-0.5X (0.022 S•cm-1), Zr0.920Y0.155Ca0.005O2.08-0.5X (0.020 S•cm-1) and Zr0.920Y0.155Sr0.005O2.08-0.5X (0.016 S•cm-1) are higher than that of (ZrO2)0.92-(Y2O3)0.08 (0.015 S•cm-1) at 800℃. The ionic conductivity of Zr0.920Y0.155M0.005O2.08-0.5X decreases with an increase of divalent ion radius. The ordering structure of oxygen vacancies occurred in the content of divalent oxides doped with ZrO2-Y2O3 higher than 0.01. The Zr0.920Y0.155Mg0.005O2.08-0.5X co-doped system contributes a maximum content of non-interfering oxygen vacancies, the average radii of co-doping divalent cations is close to that of Zr4+ and average binding energy must be as small as possible. Following these principles helps achieve the highest conductivity of zirconia. The effect of ionic radius of divalent ions is more significant than that of lattice binding energy of divalent oxides for enhancing the ionic conductivity of zirconia co-doping system.
Grain size of Zr0.92Y0.16-XMXO2.08-0.5X systems increase with an increase of divalent oxide content. It seems that the grain size is correlated with the amount of oxygen vacancy. CaO and SrO segegated in grain boundaries when CaO and SrO contents are more than 0.015mole and 0.01mole respectively, because of the ionic radius of Ca2+ and Sr2+ are larger than that of Zr4+, the solid solution of CaO-ZrO2 and SrO-ZrO2 did not form easily. This is a reason why grain growth was restrained in higher CaO and SrO content.
The radii of oxygen vacancies in co-doped system were calculated from Hard-sphere model and the results show that oxygen vacancy radius depends on the ionic radius of the divalent dapants in Zr0.92Y0.16-XMXO2.08-0.5X system, the ionic conductivity of Zr0.92Y0.16-XMXO2.08-0.5X seems to increase as the oxygen vacancy radius increased. The trend of ionic conductivity increase, which is σMgYSZ>σCaYSZ>σSrYSZ, is similar to the trend of oxygen vacancy size increase, which is rVo(Mg)> rVo(Ca)> rVo(Sr).
The ionic conductivity of Zr0.920Y0.155M0.005O2.08-0.5X possesses outstanding electrical properties which is close to that of Ce0.8Sm0.2O1.9 which is 0.037(S•cm-1). For this reason, the Zr0.920Y0.155M0.005O2.08-0.5X is suit able to apply in the reduction-resistance layer of SDC electrolyte system for sandwich structure in SOFCs and decrease the operation temperature.
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