Effects of Ca²⁺ and Zr⁴⁺ Doping on the Thermophysical Properties of Lanbo₄

Abstract

Lanthanum Niobate (LaNbO₄) doped with acceptor ions, specifically calcium (Ca²⁺) 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 LaNbO₄ for PC-SOFC applications is in the high temperature tetragonal phase (<i>t</i>-LaNbO₄), which transforms to a monoclinic structure (<i>m</i>-LaNbO₄) on cooling below ~500�C. The large change in coefficient of thermal expansion (CTE) between the high and low temperature phases of LaNbO₄, however, is a key limitation in processing dense microstructures of electrolyte membranes. As the CTE for <i>t</i>-LaNbO₄ is approximately half of that for <i>m</i>-LaNbO₄ the selection of interconnect materials for PC-SOFCs is a challenge. This study seeks to understand how doping LaNbO₄ with aliovalent ions influences the CTE and the phase transformation temperature. Part one of this study was focused on understanding how Ca²⁺ doping alters the evolution of the CTE. Part two of this study explores the inclusion of Zr⁴⁺ 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 LaNbO₄.