Effects of Ca<sup>2+</sup> and Zr<sup>4+</sup> Doping on the Thermophysical Properties of Lanbo<sub>4</sub>
Abstract
Lanthanum Niobate (LaNbO<sub>4</sub>) doped with acceptor ions, specifically calcium (Ca<sup>2+</sup>) is a potential material for proton conducting solid oxide fuel cells (PC-SOFCs). The proton conductivity of this material has been studied with promising results. Optimal proton conductivity in LaNbO<sub>4</sub> for PC-SOFC applications is in the high temperature tetragonal phase (<i>t</i>-LaNbO<sub>4</sub>), which transforms to a monoclinic structure (<i>m</i>-LaNbO<sub>4</sub>) on cooling below ~500�C. The large change in coefficient of thermal expansion (CTE) between the high and low temperature phases of LaNbO<sub>4</sub>, however, is a key limitation in processing dense microstructures of electrolyte membranes. As the CTE for <i>t</i>-LaNbO<sub>4</sub> is approximately half of that for <i>m</i>-LaNbO<sub>4</sub> the selection of interconnect materials for PC-SOFCs is a challenge. This study seeks to understand how doping LaNbO<sub>4</sub> with aliovalent ions influences the CTE and the phase transformation temperature. Part one of this study was focused on understanding how Ca<sup>2+</sup> doping alters the evolution of the CTE. Part two of this study explores the inclusion of Zr<sup>4+</sup> in the structure to decrease the transformation temperature from monoclinic to tetragonal phase. High temperature x-ray diffraction (HTXRD) was used to understand how these dopants influence the crystal structure, the phase transformation temperature, and the thermal expansion behavior of LaNbO<sub>4</sub>.
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- OSU Theses [15752]