| Summary: | 碩士 === 國立臺北科技大學 === 材料科學與工程研究所 === 99 === This investigation of doping yttrium oxide and niobium oxide to bismuth oxide based electrolyte makes the fluorite structure stable in room temperature, so as to get the solid electrolyte which exhibits good properties. The aim of this research is to develop the intermediate temperature type solid state oxide fuel cell, the lower operating temperature than the commercialized electrolyte-YSZ at present, and to reduce the cost. Without doping aliovalent ions, bismuth based electrolyte belong to the defect fluorite structure, with an attribute of having 25% vacancies in itself, a kind of defect fluorite structure, the bismuth oxide has good ionic conductivity.
By doping yttrium oxide and niobium oxide to the Bi1-xYxO1.5 (x = 0.20~0.30) and Bi1-yNbyO1.5+δ (y = 0.15~0.25), sintering at 800~950oC, then we are able to observe how properties are influenced at temperature variation. We also analyze the properties and features of structure at several conditions by XRD, SEM, Raman, TEM, DC impedance, EIS, CTE etc. After sintering we identify the specimens as the fluorite structure (Fm¯("3" )m) by XRD, then use SEM to observe the specimen, the grains grow and became more compacted as the sintering temperature rises. In YSB system, the average grain size is 0.59 μm at 30YSB/800oC, 10.63 μm at 20YSB/950oC.While in NDB system, the grain is larger than that in YSB system, the average grain size is 0.49 μm at 25NDB/850oC, 29.35 μm at 15NDB/950oC. In Raman spectroscopy analysis, we find the characteristic peak appears at wavenumber 110.7 cm-1 and 617.3 cm-1 in YSB system, in NDB system, the peaks appear at the anterior wavenumber 96 cm-1, 145 cm-1, and 740.6 cm-1 at the posterior. According to aforesaid data, we confirm that the later is the cubic-type structure. 20YSB/950oC showed the highest ionic conductivity in this DC-impedance experiment; and the conductivity of 20YSB/800oC is 0.75 S/cm and 20YSB/950oC is 1.97 S/cm at 800oC in YSB system, respectively. 15NDB/800oC shows 0.38 S/cm and 15NDB/950oC slightly up to 0.41 S/cm at 800oC in NDB system, respectively. Through EIS experiment, we are able to observe that the resistance is decreased while the estimate temperature rises up both in YSB and NDB system. In TEM microscopic analysis, we catch sight of different extra diffraction patterns by using the select area diffraction, and observe two structures– type II and type III co-exist in 25NDB/950oC specimen from high-resolution image. Examination of physical property on coefficient of thermal expansion showed that 20YSB/800oC is 22.6×10-6/oC, 30YSB/800oC is 20.6×10-6/oC, 15NDB/800oC is 19.5×10-6/oC at 600oC, respectively.
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