Preparation And Structure Of Rare Earth-doped Lanthanum Silicate Solid Electrolyte - Performance

To overcome the increasing serous energy crisis, it is very urgent to exploit new energy resources or effectively utilize traditional energies. As the energy conversion devices with high efficiency, solid oxide fuel cells (SOFCs) have attracted more and more attentions due to high conversion ratio, excellent stability and environment-friendly features. However, their high operation over800℃hinders the commercial applications of SOFCs, which not only increases the cells’cost due to the requirement for high performance materials, but also reduces the overall reliability and durability of the device. It is believed to be the inevitable trend to develop intermediate and low temperature SOFCs, which also has been faced with two new challenges: the drastic drop in ionic conductivity of cell’s materials and remarkable increase of interfacial polarization resistances between cathode/anode and electrolyte layers as the operation temperature goes down. To conquer these problems, two basic strategies have been adopted:(1) seeking novel materials with high ionic conductivity at intermediate and low temperatures,(2) exploring novel microstructures of cell components by using novel fabrication and processing techniques.To satisfy the requirement of novel solid electrolytes with high ionic conductivities for intermediate and low temperature SOFCs, apatite-type lanthanum silicate have chosen and their ionic conductivities have been enhanced by two strategies together:one is to use nano-sized powders as the starting materials to accomplish densification at relative low sintering temperature and eliminate pore defects as much as possible; the other is to be doped by rare earth, which could be help for improving the intrinsic ionic conductivities and suppressing the growth-up of crystalline during sintering to decrease the resistances from grain interfaces. Focusing on these aims, single phase un-doped and rare earth doped lanthanum silicate nano powders have been synthesized and the dense solid electrolytes have been sintered by using the synthesized nano powders as the starting materials. The following issues have been systemically investigated in this thesis:1) systematically investigating the controlling factors to improve the homogenous distributions of lanthanum and silicon elements in the gel and clarifying the possible mechanism of gelation process to establish the reliable process for fabricating single phase lanthanum silicate nano powders;2) synthesizing rare earth doped lanthanum silicate nano powders and illustrating the influences of doping rare earth on the particle size and morphologies as well as crystal structures of the synthesized nano powders;3) fabricating solid electrolytes by using the synthesized nano powders, probing the influences of doping rare earth on the grain sizes and crystal structures of the sintering electrolytes and clarifying the relationships between their micro-structures and ionic conductivities.Based on the systematical characterizations and discussions, the mainly results and conclusions were made and shown sequentially:1. The chelating of citric acid and lanthanum ions achieved to complete reaction and existed as the most stable form, which improved the homogenous distributions of the lanthanum ions in the obtained gels.2. The silicon in form of silicon gel existed as the blend with citrate gel with using tetraethyl orthosilicate (TEOS) as the silicon source, while it participated in the formation of citrate gel by amidation chemical bonding and achieved homogenous distribution at atomic level as silane coupling agent KH550was used as the silicon source.3. The supplementary oxidant, ammonia nitrate, could be effectively adjust the thermal conditions during auto-combustion process and promote the formation of target phase. The temperature homogeneities during auto-combustion were refined by using microwave ignition instead of thermal ignition, which resulted to the formation of nano powders with uniform sizes and less adhesion.4. Single phase lanthanum silicate nano powders with particle size ranging from80to100nm have been successfully synthesized with using KH550as the silicon source and oxidizing degree of60%via sol-gel auto-combustion method. The particles of the synthesized powders with slight adhesion consisted of tiny nano crystals.5. Rare earth doped lanthanum silicate nano powders with chemical formulation of La9.33-xMxSi6O26(M=Sm or Nd, x=0.00～1.00) have been directly fabricated by sol-gel auto-combustion method with using KH550as the silicon source and oxidizing degree of60%. As the doping amounts increased, the particle sizes of synthesized powders decreased and the doping element Sm or Nd played an important role in grain refinement and hindering the growth-up of the particles.6. The dense lanthanum silicate solid electrolyte with apparent density large than95%has been sintered at a relative sintering temperature of1450℃. As the sintering temperature increased, the ionic conductivities increased and reached to the highest value of0.08S/cm, which was about three times larger than the value reported in the ever references;7. The doping rare earth effective hindered the growth-up of grains during sintering process and helped for the elimination of pores to accelerate the densification. As the doping amounts gradually increased, the silica tetrahedron in the crystal structures of lanthanum silicate electrolytes have been slightly shrunk and the transport channel of oxygen ions have been enlarged, which led to the improvement of their ionic conductivities.