||In this study, we proposed sol-gel method to prepare gadolinia-doped ceria (GDC) solid electrolytes with the addition of SiOX or TiOX as the sintering aids. Materials characteristics of the prepared and calcined electrolyte samples were identified by X-ray diffraction (XRD) for crystalline structure, Specific surface area analyzer (BET) for particle size and scanning electron microscopy (SEM) for surface morphology of powders. The electric performance and density were measured by Archimedes method and AC impedance method.
For the calcined process of SiOX-doped or TiOx-doped GDC systems, the results showed that the prepared electrolyte powder had no additional impurity phases as the dopant concentrations of SiOX or TiOx were 0~3.0 wt.%. XRD results indicated lattice constants of prepared powder decreased as the dopant concentrations of SiOX or TiOx increased, because Si4+ or Ti4+ has smaller ionic radius than Gd3+ and Ce4+. For XRD and BET analyses, DBET and DXRD values showed particle sizes and crystallite sizes decreased as the dopant concentrations of SiOX or TiOx increased. The ranges of particle sizes and crystallite sizes were 22~55 nm and 20~45 nm. TGA and DTA spectra showed no obvious weight loss, and had a stable phase structure at higher temperatures (900°C). From SEM results, the agglomeration behavior of electrolyte powders for excessive SiOX-doped or TiOx-doped concentration (> 1 wt.% ) was obviously observed.
For the sintered process of SiOX-doped or TiOx-doped GDC systems, the sintered sample without additional impurity phases were obtained as the dopant concentrations of SiOX or TiOX were 0~3.0 wt.%. By SEM analysis and Archimedes method, doped 0.25~0.5 wt.% SiOX or TiOX could reduced the sintering temperature and increased the relative density. The relative density of undoped GDC pellet under the sintering process of 1500 °C and 5 hours was around 91.99 %, while the relative density of 0.25 wt.% SiOX-doped or TiOX-doped GDC pellets under the sintering process of 1400 °C and 5 hours were 91.21 % and 92.01 %. The results showed that the electrolyte with small doped concentration of SiOX or TiOX could achieve to a higher densification of electrolytes for solid oxide fuel cells (SOFCs).
For the electric characteristic, the small concentration of SiOX-doped or TiOx-doped GDC pellets had the higher conductivity than pure GDC pellets. As sintering at 1400 °C for 5 hours, the highest total conductivities were obtained in 0.25 wt.% SiOX-doped GDC pellet with σ800 °C = 2.84×10-2 S/cm and 0.25 wt.% TiOX-doped GDC pellet with σ800 °C = 5.12×10-2 S/cm, as compared to the undoped GDC with a total conductivity of 2.65×10-2 S/cm at 800 °C .
It could be concluded that the preparation of adding sintering aids in GDC electrolytes was successful to lower the sintering temperature of the densification for solid oxide electrolytes, and small concentration of sintering aids doped GDC pellets with the higher relative density and the comparable conductivity can be used as solid electrolyte layers for SOFCs system as compared to the well-known YSZ.