Experimental phase studies in the BaO-CeO2-Gd2O3 system

Energy became more important issue with developing technology. Fossil fuels play an important role to produce energy. Nevertheless, they are limiting and environmentally hazardous energy sources. Because of these reasons, energy producers have tended to use renewable energy sources such as hydropower, solar panels, biomass, geothermal energy, and wind turbines. However, there are some disadvantages of these systems; they are transient energy sources and affected so many factors such as wind speed, temperature, ambient, and weather. So they cannot be used in critical applications. At this point, fuel cells are alternative energy sources that are clean, environment-friendly and high efficient systems. Fuel cells are devices that convert chemical energy directly into electrical energy with high efficiency. In these systems, various types of fuels can be used such as hydrogen, natural gas, carbon monoxide, methane and biomass. Especially, fuel cells use hydrogen as fuel dispose water vapor as waste. This shows fuel cells have zero environmental pollutant emission. Solid oxide fuel cells are one of the most important members of fuel cells regarding to their high efficiency and operation capability with different types of fuels. However, the main problem of solid oxide fuel cell is that material selection is difficult because of high operating temperature. Thus, various types of materials have been investigated for decreasing operating temperature of solid oxide fuel cells. Gadolinium doped ceria one of the most important material for decreasing operating temperature and increasing efficiency. On the other hand, it has been shown that different dopant addition such as gadolinium, yttrium, europium, lanthanum, and neodymium improved material properties. Gadolinium doped barium cerate is a potential material for fuel cell applications since it works relatively low operating temperature and has high conductivity, high mechanical strength and mixed ion conductivity. Additionally, it has not only oxide ion conductivity but also protonic conductivity thus, it can use all the types of fuels and it has wide application areas. The purpose of this study is that experimental phase studies in BaO-CeO2-GdO1.5 quasi-ternary system, in which there is no phase diagram data available, at 1300°C with using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy analyses (EDS). Various compositions of BaCe(1-x)Gd(x)O(3-x/2) were prepared and they were named according to cerium and gadolinium amount. Conventional solid-state reaction method was used for preparing samples. BaCO3, CeO2 and Gd2O3 were used as starting powders and then they were mixed with alcohol. After that, powders were wet milled and alcohol was disposed from mixture with drying 16h at 150°C. Powders were calcined at 1100°C for 6h and then milled again for reducing particle size. Then powders were pressed at 400 MPa using cold isostatic press. Pellets were sintered at 1300°C for 48h. BCG0-67, BCG0-50 and BCG10-57 samples were sintered again at 1300°C for 72h for understanding BaGd2O4 phase formation. Moreover, BCG0-67, BCG0-50 and BCG10-57 samples were quenched for examining stability of BaGd2O4 phase. BCG0-67 and BCG25-25 samples were prepared with the Pechini (sol-gel type wet chemistry) method for crosscheck. In the sol-gel method, Ba(NO3)2, Ce(NO3)3.6H2O and Gd(NO3).6H2O powders were used as starting powders. These powders were mixed with distilled water. They were stirred and heated with magnetic stirrer. During heating one mole citric acid per cation was added as chelating agent, and then a gel formation was observed. When distilled water amount reached below a critical value, a sudden combustion reaction was observed. Then, samples were put into drying oven at 150°C for completing reaction. Organics and water were removed in drying oven and samples formed foam. Then, powders were ground and calcined at 800°C for 6 h. Calcined powders were ground again and pressed at 400 MPa in cold isostatic press thus pellets were produced. Pellets were sintered at 1300°C for 48h. Prepared samples were divided two groups, first of them was cooled in furnace and the second one was quenched. Thus, control experiment of the BaO-CeO2-GdO1.5 quasi-ternary phase diagram was completed. It was determined that BaGd2O4 is high temperature stable phase and it decomposes to BaCO3 and b-type Gd2O3. Small amount of gadolinium dissolves in barium oxide. Finally, samples were characterized with XRD and SEM/EDS thus quasiternary phase diagram of BaO-CeO2-GdO1.5 was constructed.