Effects of flash-sintering on gadolinium doped-ceria

Introduction: Gadolinium-doped ceria (GDC) is an attractive electrolyte material for IT-SOFC applications, due to its relatively high abundance and its high ionic conductivity in the intermediate temperature range (600-900°C). One of the main drawbacks of ceria-based materials is the high sintering temperature required to obtain full densification. The present work was carried out with the aim to apply the innovative Flash Sintering technique for the densification of some GDC ceramics. Methods: Gadolinium-doped ceria powders with variable dopant concentration (ranging from 5 to 20%) and different sintering aids (Li and Co) were synthesized by co-precipitation with ammonia solution of Ce and Gd nitrates. The produced nano-powders were calcined at 600°C for 1 h. All samples crystallized in fluorite habit and no phase evolution occurred by thermal heating (up to 1300°C). Powders were then shaped into pellets by uniaxial pressing and sintered under the application of an electrical field (50 V/cm and 100 V/cm). The microstructure of the flash-sintered bodies was characterized by Scanning Electron Microscopy (SEM) while the electrical behavior was analyzed by EIS spectroscopy. Results: The application of an external electric field upon heating allows to reduce significantly the sintering temperature of all considered materials. Sintering temperature is shown to decrease for more intense electrical field, even down to 400°C. The conduction behavior before upon Flash Sintering strongly depends on dopants amount. In particular, conductivity is enhanced by increasing Gd concentration or by adding lithium as sintering aid. Much lower effects on conductivity are observed by adding cobalt. In a similar way, the onset temperature for FS decreases with the powders conductivity. SEM analysis point out a nano-grained microstructure, although not all the obtained samples are fully densified. Impedance spectroscopy measurements show that the electrical properties of flash sintered bodies are perfectly comparable with values recorded on conventionally sintered GDC materials. Conclusions: Flash sintering can be successfully applied to variously-doped ceria based materials. The presence of different dopants with variable concentration influences both conductivity and the onset temperature for FS. This behavior is coherent with the model of thermal runaway for flash sintering. In spite of the very low sintering temperature (always below ca. 700°C), microstructural and impedance analysis results point out that the sintered bodies are suitable for possible application as solid electrolyte for IT-SOFC.