| Description |
To date sodium beta alumina has been known as the best sodium ion conducting electrolyte. The grain size dependent sodium ion conductivity ranges from 0.2˷0.35 Scm-1 for fine and coarse-grained samples, respectively, at 300oC. The compound has a rhombohedral structure consists of three spinel blocks separated by two conduction planes. The material has applications in a variety of technologies. The traditional processing of Na-β" Alumina is cumbersome due to the multiple high temperature steps required including encapsulation in Pt crucibles. In addition, the product is susceptible to moisture and CO2 attack due to the remaining minority phase of NaAlO2 and low strength due to the exaggerated grain growth. A vapor phase synthesis was developed to circumvent these drawbacks. In this process a two-phase sintered composite of α-Al2O3 +YSZ is converted into a Na-β"-alumina + YSZ composite. The process consists of the fabrication of a two-phase sintered composite of α-Al2O3+YSZ, exposing it to a vapor of Na2O by packing in a powder of Na-β"-alumina and heat treating at a temperature between ∼1250◦C and 1400◦C. The conversion to Na-"-alumina + YSZ occurs by a coupled diffusion of 2Na+ through the formed Na-β"-alumina and of O2- through YSZ. This dissertation presents experimental results on the characteristics of Na-"-alumina + YSZ and other composite materials synthesized by vapor phase method. It was shown that vapor phase process developed recently is not only applicable to the fabrication of Na-β" Alumina+YSZ but other compounds can also be synthesized. Specifically, starting from Ga2O3+YSZ sintered composite and conversion by vapor phase method a novel compound of sodium zirconium gallate was made. The newly made compound has a monoclinic structure with the measured conductivity of about 2.8×10-4 Scm-1 at 300oC. The second study demonstrated that transport properties of the composite fabricated by vapor phase process can be altered by doping aliovalent ions such as Fe2+ and Mg2+. Further characterizations showed that vapor-phase synthesized samples exhibit anisotropy such that in disk shaped samples, the grains (c-planes) are aligned along the disk axis. Therefore, samples showed higher (5.5 times) ionic conductivity compared to the perpendicular direction. Finally, the mixed ionic conductivity of these composite materials was studied under open circuit condition. Under sodium chemical potential difference, Nernst potential of sodium was observed, which was attributed to the ratio of Na activity on each side of the sample. Under oxygen chemical potential difference, Nernst potential of oxygen was observed, which fitted to the predicted Nernst potential of oxygen. The sample under oxygen chemical potential difference exposed to H2/H2O atmosphere was degraded. |
