Mechanochemical synthesis and electrical properties of Gd2Hf2−xZrxO7 solid electrolytes for their use in SOFC’s

Abstract

Lanthanides hafnates Ln2Hf2O7 (Ln = lanthanides) are known by their interesting electrical properties; many of them are good oxygen ion conductors at high temperatures and can be used as solid electrolytes in solid oxide fuel cells. Properties of these materials are significantly affected by the presence of defects such as vacancies and structural disorder which can be generated by single or multiple chemical substitutions or even by processing. This work analyzes the effect of Hf substitution by Zr on the electrical properties of gadolinium hafnate, starting by the synthesis of solid solutions with general formula Gd2Hf2−xZrxO7 (x = 0, 0.8, 1.2, 1.6 and 2) by mechanical milling, a powder processing method which allows obtaining metastable phases at room temperature. These phases include a large number of structural defects, which will have an interesting effect on their properties. Results show that all proposed compositions with a disordered fluorite-type arrangement can be obtained after a mechanical milling process of 30 h, by using a planetary ball mill and Yttria Stabilized Zirconia vials and balls. Post-milling thermal treatments at 1500 °C facilitates the evolution to ordered pyrochlore structures for all analyzed compositions. Their electrical properties, analyzed by impedance spectroscopy, reveal that activation energy for ionic migration increases as Zr content increases and dc conductivity, one important property for solid electrolytes application, reaches a maximum for x = 1.6 at 750 °C. This degree of doping would represent the optimal degree of disorder in the system Gd2Hf2−xZrxO7 to obtain the higher ionic conductivity.