Oxygen ion conducting pyrochlore oxides prepared by an ultrasound-assisted wet chemistry route: Ca-doped Gd2Ti2O7 nanocrystals

Different compositions of pyrochlore-type oxides of general formula Gd2-xCaxTi2O7 (x = 0, 0.05 and 0.1), interesting solid oxide ion conducting materials, were prepared by a novel wet-chemistry method assisted by ultrasound. Titanium isopropoxide (TTIP), and Gd3+ and Ca2+ hydrated nitrates were used as metal sources; whereas, ethanol and ethylene glycol were used as solvents. The results corroborate the feasibility of this method to obtain nanometric particles of lanthanide titanates. Electrical properties of uniaxially pressed and sintered pellets (1200 and 1500 degrees C) were measured as a function of temperature (280-700 degrees C) and frequency (100 Hz-1MHz) by using impedance spectroscopy. The Analysis of electrical properties revealed an increasing of dc conductivity (sigma) for higher contents of Ca2+ in the system, reaching a maximum of 3.55 x 10(-3) S cm(-1) at 700 degrees C for x = 0.1. Calculated activation energies for oxygen diffusion in the Ca-doped samples, are in the 0.68-0.89 eV range, well below that of pristine Gd2Ti2O7 (2.23 eV), which explains their enhanced conductivity. Increasing of dc conductivity is promoted by substitution of Gd3+ by Ca2+ in these titanates, generating oxygen vacancies in the structure, which facilitates the ion migration process. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.