Synthesis, Electrical Properties Of Mixed Electronic-Oxide Ionic Conducting Materials

Oxide ion conducting materials have various important technological applications. On one hand, the pure oxide ion conductors can be used for the electrolytes of solid oxide fuel cells?SOFCs?, oxygen sensors and pump. On the other hand, the mixed oxide ionic and electronic conductors can be used for cathodes for SOFCs, oxygen permeation membranes, gas conversion and reformation catalyst of syngas. The trend of lowering the operating temperature for these various applications down to 500°C has stimulated extensive search for new oxide ion conductors. In this study, Three mixed electronic-oxide ionic conducting materials were studied by doping metal ion and creating oxygen vacancies or interstitial oxygen defects. Exploring the relationship between electrical property and anionic defects in scheelite BiVO₄, layered perovskite Nd BaInO₄ and perovskite Sr FeO₃. In addition, the silicates Sr₃R₂Si₆O₁₈?R=Y, Gd?-based and double molybdate LaRbMo₂O₈ materials based on tetrahedral structures were attempted to search for some new oxide conductors. The major results are described as following:1) For BiVO₄ material, the influences of synthesis methods on conventional solid state method and laser levitation technology of Sr²⁺ substituted for Bi³⁺ was investigated. To confirm the stability and ionic migration mechanism of oxygen vacancies in Bi_1-xSr_xVO_4-0.5x compositions, the Neutron diffraction?ND?, Synchrotron radiation?SPD? and Solid-state 51 V NMR using laser levitation method were performed, and the ionic conductivity from the acceptor doping of Sr²⁺ and Na⁺ at Bi³⁺ site was studied. It's shown that the laser levitation method can widen solid solution in Bi_1-xSrxVO_4-0.5x composition, but did not make significant difference on the synthesis methods. The bulk conductivity was enhanced and the phase transition temperature was decreased by Sr²⁺ substitution for Bi³⁺. The average structure from powder diffraction using ND and SPD datas was obtained and it's proved that the V₂O₇ dimmers in Sr²⁺-doping materials by Solid-state 51 V NMR spectra. The Na⁺-doping enhanced the bulk conductivity at low temperature range only. Both Sr²⁺, Na⁺-doping had not change the band gap of BiVO₄.2) For NdBaInO₄-based material, aliovalent substitution defect chemistry was investigated to generate the oxygen defects in layered perovskite NdBaInO₄ mixed oxide-ionic and hole conductor, both the static-lattice and molecular dynamic simulations for the acceptor-doped Nd BaIn O₄ were calculated by other member in my research group. Among the alkaline-earth metal cations Ca²⁺, Sr²⁺ and Ba²⁺, the Ca²⁺ was shown to be the optimum acceptor-dopant for Nd³⁺ in NdBa InO₄ showing the largest substitution for Nd³⁺ up to 20% and leading to highest oxide ion conductivities on Nd_0.8Ca_0.2BaInO_3.9 composition. Impedance measurements under O₂/N₂ atmospheres and EMF measurements of oxygen concentration cells confirmed that the ionic conduction behavior arises from the oxide anions and the oxygen transport number in Nd_0.8Ca_0.2BaInO_3.9 composition. It's confirmed that Ca²⁺ is the optimum acceptor-dopant combining the elastic strain effect with the theoretical calculation, and the most energy-favorable vacancy migration paths were confirmed in Ca-doping Nd BaIn O₄. A small amount of Nd³⁺ can be replaced with Ce⁴⁺ to create interstitial oxygen, which has no obvious improvement on bulk conductivity of parent material. What's more, Nd_0.95Ce_0.05BaInO_4.025 isn't suitable for used as cathode material because the lower conductivity in 5%H₂/Ar atmosphere. La³⁺/Nd³⁺ substitution for Ba²⁺ or Nb⁵⁺ substitution for In³⁺ were found to be very difficult for generating interstitial oxygen defects in NdBa InO₄.3) In reversible oxidation-reduction phase transition of SrFeO₃, Zr⁴⁺ was substituted for iron ion to stabilize cubic perovskite phase, and conductivity and area specific resistance were measured. La³⁺/Pr³⁺ substitution for Sr²⁺ were performed based on the reduction phase of the brownmillerite Sr₂Fe₂O₅ structure for creating the interstitial oxygen defects and improve the conductivity of material. The results show that the short-term stability of cubic phase was found to reduced atmosphere by doping a small amount of Zr⁴⁺ and the polarization resistance needed to be improved. The polarization resistance of SrFeO₃ was decreased via regulating the content of ethyl cellulose appropriately, and the enhancement of conductivity in La³⁺/Pr³⁺-series compositions were confirmed in 5%H₂/Ar, in spite of the high polarization resistance in materials.4) In order to search for the new oxide ionic conductors based on tetrahedral structures, Na+ substitution for Sr²⁺ in Sr₃R₂Si₆O₁₈?R=Y, Gd? containing isolated [Si₃O₉] rings were carried out for introducing oxygen vacancies, meanwhile, the mole ration of Sr²⁺ and Y³⁺(or Gd³⁺)were tunned for creating the oxygen defects. Also, we have tried to synthesize the composition for double molybdate LaRbMo₂O₈. The results show that a) The conductivity of Sr₃Y₂Si₆O₁₈ was higher than Sr₃Gd₂Si₆O₁₈ sample, For Sr_3-xNa_xY₂Si₆O₁₈?x=0.3, 0.5? compositions, the phase formation temperature were decreased and the conductivities were enhanced effectively by Na+ doping for Sr²⁺. b) the tetragonal phase structure LaRbMo₂O₈ powder was achieved at 800°C for 10 hours, and the compound undergo a phase transition from tetragonal phase to an unknown new structure above 580°C at heating process, which become purely tetragonal phase at 150°C at cooling process.