Development of SOFC single cell using 8YSZ as an electrolyte component is one of the major issue. We developed a new electrolyte material for SOFC using Bi2O3 as dopant and sintering aid to 8YSZ for decreasing the sintering temperature and increasing electrical property especially at ionic conductivity. Bismuth oxide added yttria stabilized zirconia as electrolyte material for SOFC was investigated using x-ray diffraction, scanning electron microscopy, transmission electron microscopy and AC impedance spectroscopy.
It was found that the small addition of Bi2O3 was effective in reducing the sintering temperature and promoting the densification rate of the ceramics. At lower sintering temperatures, Bi2O3 phase is found to act as a wetting agent of the grain boundaries. The majority of Bi2O3 was segregated at the grain boundaries which formed Bi2O3 - rich liquid films on the basis of STEM/EDS analysis and the thin Bi2O3 -rich liquid film is found to serve as a diffusion path for the sintering.
XRD analysis reveals the coexistence of cubic and monoclinic phases in pure 8YSZ and Bi2O3 doped 8YSZ at lower sintering temperatures. But, at higher sintering temperatures, 8YSZ electrolyte and low content of Bi2O3 doped 8YSZ electrolyte show pure cubic phase in micro-scale observation. This might be due to appropriate sintering temperature and Bi2O3 content.
Microstructural study shows bimodal grains. Bismuth rich is found in smaller grains. Transmission electron micrographs revealed the similar behavior. But, the diffraction pattern of big grain show cubic phase and smaller grain resulted in monoclinic phase. Though the monoclinic phase is not observed in XRD analysis, the nanoscale analysis using TEM has shown the existence of monoclinic phase even at higher sintering temperatures.
AC impedance analysis indicates better grain boundary conduction in doped 8YSZ. Grain boundary conduction was significantly improved in our observation. The total conductivity of 8YSZ was evidently increased 26% compare to pure 8YSZ by doping small amount of Bi2O3. The change in conductivity by the addition of dopant Bi2O3 in these material systems was correlated with microstructure analysis. The experimental results demonstrate better density, improved microstructure, higher ionic conductivity and lower activation energy in bismuth oxide doped 8YSZ electrolytes compared to that of pure 8YSZ. The results on conventional sintered samples indicate that the minimum processing temperature needed to stabilize the cubic phase is above 1300oC for 8YSZ and 0.5mol% Bi2O3 added 8YSZ to use these materials as electrolytes in the solid oxide fuel cells.
Microwave sintering was also carried out on microwave sintering in order to reduce the evaporation of Bi2O3 rate during sintering process. Microwave sintering is found to suppress the evaporation rate of Bi2O3. Compared to conventional furnace, significant improvement in density of zirconia ceramics at lower sintering temperature, rapid heating rate and short sintering time were observed. Microwave sintering was proved to overcome the problems with the evaporation of Bi2O3 during sintering process. By using microwave sintering at the same sintering temperature the total density of 8YSZ+10% Bi2O3 was evidently increased 4.59 % in comparison to YSZ+10% Bi2O3 sintered with conventional furnace and the total ionic conductivity was enhanced by 6.578 % in microwave sintering process.

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