Preparation And Characterization Of La_0.8Sr_0.2Co_1-yFe_yO₃ For Cathode By PVA Method

Solid Oxide Fuel Cell (SOFC) is a solid-state device for direct conversion of chemical energy into electrical energy. Perovskite-type complex oxides of La_1-xSr_xCo_1-yFeyO₃ are considered promising cathode materials because of superior mixed electronic-ionic conduction properties. In this dissertation, a new liquid-phase-method—PVA method having indutrial feasibility has been put forward,Its purpose lower cost of powder preparation and make it possible for industrial application. La_1-xSrxCo_1-yFeyO₃ nanometer-size powders have been synthesized by citrate method and PVA method.It was found that the ratio of citric acid to total metal cation content(abbreviated as citr-/Mn+),pH of sol,temperature of preparation for gel influence on the dry gel,component of powders and structure of crystal structure by citrate method and the ratio of total metal cation charge to secondary material A(Mn+/Ax-),DP of A,temperature of preparation for gel pay a key role in determining the properties of the dry gel and powders by PVA method.The processing parameters were studied through experiments. Compareing with citrate method, PVA method is better because of low cost, short production cycle, maneuverability. Synthetical mechanism has been studied by Thermogravimetric analysis (TG-DTA),Fourier transform infrared spectroscopy(FT-IR),X-ray diffraction (XRD).It was found that nitrate and hydrocarbon decomposed partly,releasing lots of CO₂,at the same time mid-phase SrCO₃ formed because of reaction between CO₂ and SrO. Nitrate and hydrocarbon disappeared till 600℃. Single perovskite formed at 800℃ for 2 hours.The powders have a structure of perovskite through XRD.The size of powders by PVA method is homogeneous through Scanning electron microscope(SEM),Transmission electron microscopy (TEM) while in large range by citrate method.But the size of powders through two methods is nanometer-size. The electrical conductivity and ionic conductivity of La_1-xSr_xCo_1-yFeyO₃ ceramics were studied by volt-ampere method and impedance spectroscopy(IS) method respectively. It was found that electrical conductivity rise with increasing of sintering temperature and Co-content.The electrical conductivity has peak value with temperature increasing, The electrical conductivity rise slowly at low temperature because the conduction mechanism of the cathode material can be described as a small-polaron hopping process while reduce at high temperature because conduction mechanism is oxygen vacancies. The ionic conductivity rise with increasing of temperature and Co-content.Charge displacement control the reaction of electrode at low temperature while gas diffusion and charge displacement control the reaction of electrode together at high temperature.The electrical conductivity is 386 S?cm-1 at 800℃ and ionic conductivity is 0.043 S?cm-1,these can meet the use of SOFC. It was found that there is not new material by sintering mixed La0.8Sr0.2Co1-yFeyO3 and Ce0.8Gd0.2O1..9(GDC) at 1200℃ for 10h.The coefficients of thermal expansion (TEC) rise with increasing of temperature and reach peak value and it is between 19.4×10-6cm/cm℃ and 24.9×10-6cm/cm℃.Posrsity is between 9.57% and 10.69% and rise with increasing of Co-content.Density is between 89.31% and 90.43% and reduce with increasing of Co-content.