Chemical Method Synthesis, Structure And Properties Of La₂NiO_4+δ Based Compounds With A Layered-structure

Recently, oxygen hyperstoichiometric La₂NiO_4+δ based compounds with a K₂NiF₄ structure have attracted growing attention as a novel mixed electronic-ionic conductor. These materials exhibit high oxygen diffusion and surface exchange coefficients at intermediate temperatures (600-800 ℃) together with moderate thermal expansion coefficients around 13.0×10^-6 K^-1, making them a promising candidate material for cathode of intermediate temperature Solid Oxide Fuel Cells (SOFCs). In this dissertation, a polyaminocarboxylate complex method has been developed to synthesis La₂NiO_4+δ based compounds. The influence of composition and sintering condition on the structure and mixed conducting properties of the resulting La₂NiO_4+δ based ceramics have been investigated. The contribution of change in crystal structure, microstructure and electronic structure to the mixed conducting properties of La₂NiO_4+δ based ceramics has been examined. La₂NiO_4+δ based compounds with a pure K₂NiF₄ phase have been derived from a polyaminocarboxylate complex precursor with diethylenetriaminepentaacetic acid (H₅DTPA) as ligand. The thermal decomposition behavior of the polyaminocarboxylate complex precursor and the formation process of the desired product were investigated. It has been found that the mole ratio of total metal cation to H₅DTPA and calcining temperature play a determinant role on the phase development and morphology of the resulting powders. A mole ratio of total metal cation to H₅DTPA of 3:1.7 was determined to be moderate in synthesizing La₂NiO_4+δ based compounds. A bidentate coordination of the carboxylate groups to metallic cations was certified for the amorphous polyaminocarboxylate complex precursor. The K₂NiF₄ phase appeared in addition to small amounts of intermediate phases after calcining the precursor at 700 ℃. Fine and uniform powders (100-200 nm) with a pure K₂NiF₄ phase can be obtained by calcining the polyaminocarboxylate complex precursor at 900-950 ℃ for 2h in air, providing an advantageous alternative to the conventional solid state reaction route in synthesizing La₂NiO_4+δ based compounds.The influence of sintering temperature on the structure and mixed conducting properties of the resulting La₂NiO_4+δ based ceramics was investigated. It has been proved that the microstructural evolution and stoichiometrical change of the specimens during sintering are responsible for the variation in the mixed conducting properties with sintering temperature. Therefore, adequately controling sintering temperature was considered to be crucial in order to achieve desired mixed conducting properties. With respect to the mixed conducting properties, the preferred sintering temperature was ascertained to be 1300 ℃ for La₂NiO_4+δ. La₂NiO_4+δ ceramic sintered at 1300 ℃ gives a maximum electrical conductivity of 102.9 S·cm^-1 at 450 ℃ and electrical conductivities of 75.5-95.3 S·cm^-1 at 600-800 ℃, which are obviously higher than those of the specimen made by the...