Study On The Precursor Systems Of Doped Lanthanum Chromite Prepared By Inorganic-organic Sol Methods

With the extensive use of doped Lanthanum Chromites (LaCrO₃) in Solid Oxide Fuel Cells (SOFCs), the improvement progress of preparation methods to get superfine powders were accelerated. The inorganic-organic sol has significant application prospect because of the uniformity and the stability of the superfine powders made by the method.The thesis introduces the recent advances on the techniques of preparation of inorganic powders, systemically gives an over view of the influences of different chelating-agent systems on the process of preparation, on the structures and the shapes of ultimate powders in the wet-chemical methods. On the above bases, the doped Lanthanum Chromites (LaCrO₃) superfine powders were prepared via the inorganic-organic sol process, and disodium ethylene diaminetetraacetate (EDTA-2Na),citric acid (CA),ethylene glycol (EG),urea which structure from complex to easy were used as the ligands in the precursor. The differences among these singlesite-chelating-agent systems and dual-chelating-agent systems were commented, otherwise, the effects on addition of ammonium nitrate to the precursor were studied. Fourier transform infrared spectroscopy (FT-IR), thermal analysis (TG-DSC and TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution electronic microscopy (HREM) were used to study the precursor structure, the processing of heat decomposition, the morphology of the powders and the structure of crystals. A preference chelating-agent system was got by comparing different precursors' effection on the ingredient and morphology of powders.In this inorganic-organic sol method, the ligands of complex structure (EDTA-2Na and CA),straight chain structure (EG) and micromolecule structure (urea) can all coordinate with metal ions, but their coordinating capability and the dispersity to metal cations decrease successively in the singlesite-chelating-agent systems. Furthermore, as the chemical bonds formed by esterification of hydroxyl with carboxy in CA are much stronger than the hydrogen bonds in EDTA-2Na system, the CA is a preferable chelating agent in the singlesite-chelating-agent systems. Even the EG and urea systems have coordinating capability but can't get uniform and steady sol. After combustion, powders in the main phase of LaCrO₄ were gained in EG system, which need a further heating to get perovskite-type structure.With a low-grade cross linking in dual-chelating-agent system of EG and urea, some improvements were made to the uniformity of the precursor, but still a uniform and stable sol couldn't be got because of the restriction of both molecular structures. However, the precursor is a rich-combustion system that can yield perovskite type LaCrO₃ directly at a lower temperature. The dual-chelating-agent system of CA and EG is a perfect system to get LaCrO₃, in which the esterification of EG and CA form a large network configuration, and the addition of EG achieves the uniformity and stability to ions distribution. Furthermore, the problem that the coordinating capability to Ca is small by other ligands was solved in the dual-chelating-agent system of [EDTA-CA-EG], which is the ideal dual-chelating -agent system to prepare Ca doped LaCrO₃.Depending on the existence of oxidant (ammonium nitrate), the inorganic -organic sol reaction can be divided into two different kinds: the Pechini method and the low-temperature combustion method. The decomposition of gels in Pechini method is a chain reaction process. In the combustion method, the abundant ammonium nitrate added to the reaction system made the organic groups decompose spontaneously at low temperature, so the combustion method was actually a kind of self-propagating combustion synthesis. The lanthanum chromites prepared by two methods have the same perovsike crystal structure, the grain size of the powders prepared by low-temperature method was smaller and has lighter agglomerate.