Electrical and thermal properties of LT-SOFC solid electrolytes: Sm cerates/zirconates obtained by mechanochemistry

In this research, advanced ceramics based on samarium cerates/zirconates of general formula Sm_xCe_1-y-xZr_yO_2-δ (x = 0.25, y = 0, 0.25, 0.5, and 0.75; x = 0.4, y = 0, 0.2, 0.4, and 0.6) were synthesized by mechanochemistry, and their electrical and thermal properties were analyzed for their potential application as solid electrolytes in Low-Temperature Solid Oxide Fuel Cells (LT-SOFC). High-purity oxides of the involved elements, used as starting reagents, were milled for 20 h in a planetary ball mill to synthesize all the studied compositions. The evolution of samples with milling time and after be fired at 900–1500 °C was evaluated by XRD. The milled samples were uniaxially pressed and sintered at 1500 °C to evaluate their microstructures by FE-SEM. Electrical properties were studied in sintered pellets (1500 °C) by Impedance Spectroscopy at different temperatures (300–650 °C) and frequencies (100 Hz - 1 MHz) to evaluate the conductivity and activation energy of all systems. Thermal conductivities were calculated from experimental data of thermal diffusivity (Laser Flash Method), density (Archimedes Method), and heat capacity (DSC) of each system. The obtained results corroborated the viability of mechanical milling (20 h) to obtain samarium cerates/zirconates at low temperatures. XRD analysis revealed the generation of cubic phases with fluorite or pyrochlore structure, even after thermal treatments at 1500 °C. Results of electrical properties corroborate the viability of these materials to be used as solid electrolytes in LT-SOFC, since the incorporation of CeO₂ promotes an increase in the ionic conductivity, reaching values as high as 1.007 × 10 ⁻¹ and 7.2 × 10 ⁻³ Scm⁻¹ (650 °C). Thermal conductivity of the samples was found between 0.74 and 1.9 W/m K (700 °C). Analysis of electrical properties revealed a trend towards an increase in ionic conductivity for compositions with higher contents of Ce⁴⁺, while the thermal conductivity decreases with a higher percentage of Zr⁴⁺.