Chemical synthesis of multi-cation oxide powders for solid oxide fuel cell (SOFC) components
Öncel, Çınar (2003) Chemical synthesis of multi-cation oxide powders for solid oxide fuel cell (SOFC) components. [Thesis]
This study involves the synthesis of LSGM (Lao.gSro.iGao.sMgo.aOs-a), LSFM (Lao.9Sr0.iFe0.8Mgo.203-5), and LSCM (Lao.gSro.iCro.sMgtuOs-s) powders via organic precursor method by using different organic carrier materials, investigation on the effects of each organic carrier material on the intended and unwanted phase formations, analyses of formed phases during stages of synthesis, characterization of the synthesized powders, crystallographic studies on the several new crystal phases, the effects of holding time during powder calcination, and further work advices. Citric acid, tartaric acid, Pechini precursors, polyvinyl alcohol, and ethylene diaminetetraacetic acid were used as organic carrier materials. Different organic carrier materials exhibited different behavior on the synthesis of powders. Synthesis of powders without carrier materials was conducted and the effectiveness of organic carrier materials was confirmed. In the LSGM synthesis, the effects of different starting materials (namely lanthanum chloride or gallium sulfate) were also investigated. X-ray powder diffraction measurements showed that unwanted phases formed, especially below 1000°C. In powders heat treated at low temperatures (< 1000°C), maximum LSGM concentration was 88% when citric acid was used as the organic carrier material. Above 1000°C, maximum concentration of LSGM phase in the powders was 95.7% when tartaric acid was utilized as the organic carrier material. For low temperature (below 1000°C) synthesis citric acid, and for above-1000°C synthesis tartaric acid are the best organic carrier in terms of LSGM percentages in the powders. It was shown that increasing dwell time at calcination temperature could increase the concentration of the desired phases in the powder. The powder synthesized with PVA as the organic carrier material was calcined at 1100°C and LSGM phase in the powder was 33.7%. When same powder held 7 hours at the calcination temperature, LSGM phase in the powder increased up to 79.8%. Single-phase LSFM was obtained in the powders calcined as low as at 550 C. In contrast to LSFM, maximum concentration of LSCM phase in the synthesized powders was 96.9%, when polyvinyl alcohol (PVA) was the organic carrier material. The factors affecting the purity of the desired phase were stated as the type of the organic carrier material, its cation chelating and/or complexing ability, and the interaction of the functional groups with the constituent cations. The necessity for further studies the organic carrier - cation interaction highlighted. The structures of La4Ga209 and LSCM were discussed in light of the observed shifts in the peak positions in the x-ray spectra.
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