M.S. (Master of Science)
Department of Physics
This thesis presents the results of investigation of the (Y,Ce)MnO₃ system and the phase diagram of the (R[sub 1-x]R[sub x])-(In[sub 1-y]Mn[sub y])O₃ system for the stability of the hexagonal structure and the presence of excess interstitial oxygen content for the improvement of the electrolyte materials of solid oxide fuel cell. Various methods of the materials synthesis, structure investigation by X-ray diffraction, oxygen content change by high pressure annealing, oxygen content determination by thermogravimetry analysis, and the collaborative ionic conductivity measurement were conducted to achieve new electrolyte materials with promising properties. The investigation of the Y[sub 1-x]Ce[sub x]MnO[sub 3+delta] samples at the solubility limits of the hexagonal and the perovskite structures found single hexagonal phase Hex0 (the space group P6₃cm) forming in Ar up to x = 0.2. Annealing in pure and high-pressure oxygen established the enhanced stability ranges of the interstitial oxygen excess in material to 1000°C. Careful thermogravimetric measurements in oxygen and hydrogen for selected Y[sub 0.9]Ce[sub 0.1]MnO[sub 3+delta] sample demonstrated presence of oxidized Hex1 and the new Hex3 phases while Hex2 was never observed. These measurements confirmed that Ce³⁺ is present in Y[sub 1-x]Ce[sub x]MnO₃ during the synthesis in Ar at 1400°C and hydrogen at 400°C. The Y[sub 1-x]Ce[sub x]MnO₃ compounds are promising materials for development of intermediate temperature electrolytes. The experimental phase diagram of the (R[sub 1-x]R'[sub x])-(In[sub 1-y]Mn[sub y])O₃ system was selected based on literature data and the predictions of stability of the hexagonal structure by using tolerance factor. The goal was to enhance properties of the electrolyte material by reducing the possible electronic conductivity of the Y[sub 1-x]Ce[sub x]MnO₃ compounds by dilution of the Mn-O network with insulating In-O, while preserving the hexagonal structure and presence of the excess oxygen above 400°C. The border line between the two types of the x-ray diffraction patterns of the isostructural hexagonal Hex0 structure based on the Mn and In was determined at y [greater than or equal to] 0.5 and y < 0.5, respectively. It was found that it was not possible to substitute Ce for the Hex0-In structure. However, the Hex0-Mn samples containing In (y = 0.5 and 0.75) could be substituted with Ce and showed large amounts of excess oxygen up to 3.25. This result constitutes the major discovery of this Master thesis work. Three most promising samples, Y[sub 0.9]Ce[sub 0.1]MnO[sub 3+delta], Tbsub .92]Ce[sub 0.08]In[sub 0.5]Mn[sub 0.5]O₃, and DyIn[sub 0.25]Mn[sub 0.75]O₃, have been sent to collaborators, and the ionic measurements of these material are in progress in the collaborator's institutions (unfortunately, the results have not received to date).
Allahyani, Wadiah, "Oxygen storage and electrolyte materials of RMnO₃" (2017). Graduate Research Theses & Dissertations. 4240.
Northern Illinois University
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