Mechanisms and stability of oxide-ion transport in homogenous and heterogeneous ceramic membranes.

摘要

Solid oxide-ion conductors are basic components of several modern technologies. Oxide-ion electrolytes are oxide-ion conductors and electronic insulators; they are used in oxygen sensors and solid oxide fuel cells. The required oxide-ion conductivity is only achieved at higher temperatures. Commercialization of this technology demands the development of a better oxide-ion electrolyte and/or the ability to fabricate a large area ceramic membrane with a thickness of L 10 μm. This thesis addresses the development of an improved oxide-ion electrolyte and of mixed ionic-electronic conductors that can be used as the electrodes of a fuel cell. The perovskite LaGaO₃ doped with Sr and Mg was known to be a superior oxide-ion electrolyte. Optimization of the composition in a narrow single-phase field was accomplished, and its stability against mixed-conductor electrodes was engineered and demonstrated in tests on a single fuel cell.; In addition to fuel-cell electrodes, mixed ionic-electronic conductors are used as gas separation membranes and methane conversion reactors that produce syn-gas. Structural and chemical stability of mixed conductors are a major problem for ceramic-membrane reactors because the material must exhibit good mixed conduction in both high and very low oxygen partial pressures and at operating temperatures, 600°C ≤ T_op. ≤ 900°C. The material SrMnO₃ is a high-temperature, oxygen-deficient, perovskite that may be preserved at room temperature. Although this material exhibits good mixed conduction, it reverts to its stable stoichiometric phase under oxidizing operating conditions. La₂NiO_4+δ has a tetragonal crystal structure that is closely related to the cubic perovskite structure. The ionic conduction occurs via the migration of interstitial oxygen, which is lost in reducing atmospheres. The stability of mixed conduction within one material proved difficult to achieve in both reducing and oxidizing conditions at high temperatures.; Several oxides are known to exhibit stable ionic conduction in membrane operating conditions. A noble metal can provide a pathway for electronic conduction while the oxide phase conducts the oxygen ions. This heterogeneous composite configuration improves stability, but the exact nature of the conduction processes has not been determined. The performance of two composite materials, Ce _0.8Sm_0.2O_1.9/Pd and (Bi_1.75Y_0.25 O₃)_0.95(CeO₂)_0.05/Ag, was assessed through permeation studies.