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dc.contributor.advisorIrvine, John T. S.en
dc.contributor.authorLashtabeg, Annaen
dc.coverage.spatialv, 243 p : ill. (some col.) 30 cm.en
dc.description.abstractIn this work I present the results of my studies on a series of reduced niobium based rutile structures: Nb₂TiO₇ doped with Fe and Zr, Ti₁₋₂ₓCrₓNbₓO₂ solid solution series. Strontium niobates of the stoichiometry Sr₄Nb₂O₉, Sr₂Nb₂O₇, Sr₂Nb₂O₆ and their reduced phases were also investigated, along with CaNb₂O₆ and BaNb₂O₆. Thermal expansion and electronic conductivity of these materials were investigated under oxidising and reducing conditions. Nb₂TiO₇ goes to rutile structure Nb ₁.₃₃Ti₀.₆₇O₄ under reducing conditions, and this has the highest conductivity of all materials investigated at 300 Scm⁻¹ at 900°C with p(O₂)=10⁻²⁰ atm, but the lowest thermal expansion of 3.00±0.05x10⁻⁶ K⁻¹ (100°C-900°C), which is incompatible with the thermal expansion coefficient of the Ni/YSZ anode in the solid oxide fuel cell of 10.3x10⁻⁶ K⁻¹. Doping Nb ₁.₃₃Ti₀.₆₇O₄ decreases its conductivity, but increases its thermal expansion to a maximum of 6.3x10⁻⁶ K⁻¹ for Nb₁.₃₄₇Ti₀.₆₃₉Fe₀.₀₀₁₄O₄. The Ti₁₋₂ₓCrₓNbₓO₂ solid solution series shows a maximum thermal expansion of 8.5x10⁻⁶ K⁻¹ for x=0.1 which then drops with increasing x to 5.6x10⁻⁶ K⁻¹ for x=0.5. The conductivity of these samples, however, reaches a maximum of ~20Scm⁻¹ at p(O₂)=10⁻²⁰ atm at 900°C for x=0.1-0.3, then drops to ~10 Scm⁻¹ for x=0.4 and -6 Scm⁻¹ for x=0.5. Sr₄Nb₂O₉, Sr₂Nb₂O₇, SrNb₂O₆, CaNb₂O₆, and BaNb₂O₆, all show fairly poor electronic conductivities in air and 5%H₂/Ar (p(O₂)=10⁻²⁰ atm) at 900°C which make them unsuitable for use in the anode of the SOFC. Structurally they are very stable to reducing conditions up to 1200°-1300°C in 5% H₂/Ar compared to Nb₂TiO₇ whose structure changes to rutile upon reduction. In the systems studied, there are two types of conductivity that dominate. BaNb₂O₆ or Sr₂Nb₂O₇ show a linear dependence over the p(O₂) range with simple defect equilibrium and fair kinetics. The rutile Ti₁₋₂ₓCrₓNbₓO₂ series, on the other hand, shows complex phase transitions throughout the p(O₂) range with kinetically limited reduction.en
dc.publisherUniversity of St Andrewsen
dc.subject.lcshSolid oxide fuel cellsen
dc.subject.lcshNiobium compoundsen
dc.titleNiobium based materials for use as current collectors in the anode of solid oxide fuel cellsen
dc.type.qualificationnamePhD Doctor of Philosopyen
dc.publisher.institutionThe University of St Andrewsen

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