| Summary: | 碩士 === 逢甲大學 === 材料科學所 === 98 === In the present study, ceria-based oxygen sensor has been prepared from the precursors of cerium (III) nitrate hydrate (CeNH) and cerium (IV) ammonium nitrate (CeAN) by spray pyrolysis (SP) and following with a screen printing technique. Zirconium acetate and yttrium acetate were doped into the cerium precursors to result in the zirconia-doped ceria (ZDC) and yttria-doped ceria (YDC) powders during SP. The effects of the zirconia and yttria doping on the properties of ceria were investigated. Moreover, gadolinium oxide and samarium oxide were also doped into the CeNH-derived ceria to produce GDC and SDC powders for the evaluation of oxygen sensing properties.
The microstructural observation showed that the ceria particles derived from CeAN and CeNH precursors exhibited respectively solid and hollow structures. The mean particle size of solid particles was found to be smaller than that of hollow particles. And, the grain growth of ceria powder tended to be inhibited by doping. The lattice constant of ceria was decreased when doping yttria and zirconia, whereas; increased when doping gadolinia (Gd2O3) and samaria (Sm2O3). The screen-printed coatings on the alumina substrate showed different particle dispersion behaviors. The CeAN-derived ceria coating exhibited an island-like structure due to its poor dispersibility in screen-printing paste. The CeNH-derived ZDC powders exhibited a better dispersion behavior, showing a 3-D network structure after screen printing.
The electrical conductivity data showed that the undoped ceria powder exhibited an n-type semiconductance. In higher oxygen partial pressure region, an ionic conductance of the undoped ceria was found to show a flattened tendency in the log σ?{ Po2 diagram. With decreasing the oxygen partial pressure, the aliovalent-doped ceria showed a wild ionic conductance, and then showed an n-type electronic conductance in lean oxygen partial pressure region. Furthermore, samaria-doped ceria (SDC) even exhibited a p-type semiconductance in the high oxygen partial pressure region. No ionic conductance was found in ZDC coating in the entire oxygen partial pressure region. It also showed higher conductivity than the undoped ceria. According to the relation of R ?f Po2?{1/n, the n value ranges in 4?{6 for a semiconductive ceramic without ionic conductance. The CeNH-derived ZDC was estimated to reveal an n value in 4-6. Additionally, all the ceria-based sensors derived from both the CeNH and CeAN possessed higher values than such range due to the minority presence of ionic conductance. In the dynamic response data, CeNH-derived ZDC sensor exhibited the shortest response time to the change of oxygen partial pressure. The higher the operation temperature of the sensors, the shorter the response time.
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