Browsing by Author "Solak, Nuri"

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    Interface stability in solid oxide fuel cells for intermediate temperature applications
    (2007) Solak, Nuri; Aldinger, Fritz (Prof. Dr. rer. nat)
    Strontium- and magnesium-doped lanthanum gallate (LSGM) perovskite-type compounds and doped ceria-based materials have recently been considered the most promising solid electrolytes for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. While nickel metal is commonly used for the fabrication of cermet-type anodes, the rare earth nickelates, such as Sr-doped La2NiO4 (LSN), are recently developed high-performance cathode materials. For successful implementation in IT-SOFC, it is therefore essential to know the phase equilibria and thermodynamic properties for systems representing the solid electrolyte and electrode materials across their various combinations. This thesis aims to determine the phase equilibria and the thermodynamics of the relevant phases in the systems La-Sr-Ga-Mg-Ni-O, Ce-Gd-Sr-Ni-O, and Ce-Gd-La-Ni-O. Subsystems of these multi-component systems were thermodynamically modeled, based on the available literature and experimental data obtained from this work. The experimental studies were designed based on the calculated phase diagrams. A minimum number of compositions was chosen strategically to obtain a preliminary prediction of the phases in equilibrium in each constituent subsystem. Finally, the experimental and computational results were used to predict the compatibility/reactivity of IT-SOFC components under fabrication and/or operation conditions. Various experimental techniques were employed for determination of the phase equilibria such as Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX), X-ray Diffraction (XRD), Differential Scanning and Adiabatic Calorimetry, and Mass Spectrometry (MS). The CALPHAD-method (CALculation of PHAse Diagrams) and THERMOCALC software were used to obtain self-consistent sets of Gibbs energy functions. The following systems were investigated experimentally: La-Ni-O, La-Ga-Ni-O, La-Sr-Ni-O, La-Mg-Ni-O, La-Ga-Mg-Ni-O, La-Sr-Ga-Ni-O, La-Sr-Ga-Mg-Ni-O, Ce-Ni-O, Ce-Sr-O, Gd-Ni-O, Gd-Sr-O, Ce-Gd-Ni-O, Ce-Gd-Sr-O, Ce-Sr-Ni-O, Gd-Sr-Ni-O, Ce-Gd-Sr-Ni-O and Ce-Gd-La-Ni-O. Using results from this experimental work and data from the literature, the following systems were thermodynamically modeled: La-Ni-O, La-Ga-Ni-O, La-Sr-Ni-O, La-Mg-Ni-O, Ce-Ni-O, Ce-Sr-O, Gd-Ni-O and Gd-Sr-O. The systems, La-Ga-Mg-Ni-O, La-Sr-Ga-Ni-O, and Ce-Gd-Ni-O were extrapolated using parameters optimized from the constituent lower-order systems. In the La-Ni-O system, the enthalpy of formation, entropy and heat capacity of La3Ni2O7, La4Ni3O10, and LaNiO3, were determined experimentally for the first time using equilibration with the gas phase, adiabatic calorimetry and differential scanning calorimetry. In the La-Ga-Ni-O, La-Sr-Ni-O and La-Mg-Ni-O systems, extended solid solutions of La(Ga,Ni)O3, La2(Ni,Ga)O4, La4(Ni,Ga)3O10, (La,Sr)2NiO4, and La2(Ni,Mg)O4 were found, and the limits of their homogeneity ranges have been established for the first time. In addition, the compound LaNiGa11O19, with a magnetoplumbite-type structure was identified, which has not been reported in the literature to date. In the La-Ga-Mg-Ni-O system, the temperature dependence of the quasi-quaternary homogeneity range of La(Ga,Mg,Ni)O3 was determined. In the La-Sr-Ga-Ni-O system, a reaction was observed between LaGaO3 and LaSrNiO4 that formed a melilite-type La1-xSr1+xGa3O7+z, LaGaSrO4 and NiO phase. Similar reaction mechanisms were observed in the La-Sr-Ga-Mg-Ni-O system. Experiments in the Ce-Ni-O system were conducted in air as well as in a reducing atmosphere. It has been found that NiO does not react with CeO2. In the Ce-Sr-O system, the entropy and heat capacity of Sr2CeO4 were experimentally determined for the first time. In the Gd-Ni-O system a eutectic reaction was observed (liquid <=> B-Gd2O3 + NiO). The Gd-Sr-O system was modeled thermodynamically based on data from the literature and the experimentally determined homogeneity range on the Gd2O3-rich site. In the Ce-Sr-Ni-O system the solid solution of (Ce,Sr)2NiO4-z was determined. No reaction between NiO and SrCeO3 / Sr2CeO4 was found. Similarly, in the Ce-Gd-Ni-O system, no reaction was observed between (Ce,Gd)O2-z and NiO. In contrast, solid solutions of Sr(Ce,Gd)O3, Sr2(Ce,Gd)O4 and (Gd,Sr)2(Sr,Ce)O4 were determined in the Ce-Gd-Sr-O system. Also, an extended solid solution of (Gd,Sr)2NiO4 was found in the Gd-Sr-Ni-O system that does not exist in the quasi-binary sections, but is stable in higher-order systems only because a solid solution is formed. It has been also found that there is no NiO solubility in the Gd2SrO4 phase. It could be concluded that doped ceria-based materials are chemically compatible with NiO during conditions typical for both the fabrication and the operation of IT-SOFC’s, whereas LSGM-type electrolytes react with NiO under the fuel cell fabrication conditions. Moreover, although La2NiO4 is a high-performance cathode, it cannot be used in combination with LSGM- or CGO-type electrolytes, due to its reactivity with both of these materials under fabrication conditions.