Yttria-stabilized Zirconia - Preparation Of Silicate Lanthanum Composite Oxide Ion Conductors, Electrical Conductivity And Related Mechanism

Nano-composite materials combine the merits of nano-technology and composite technique, and they always show a unique performance that is better than the ones of the components. The properties of nano-composite materials are changed for the higher volume ratio of interfaces and a large amount of interface defects. The carrier redistributed in space charge region and lattice misfit produced at interfaces, and these changes play a significant role in the enhancement of conductivities. Therefore, the nano-composite method has been being applied in researching and development of functional materials, and supplied a novel way to improve the conductivities of oxygen-ion conductors.8 mol% yttria stabilized zirconia (YSZ) and lanthanum silicates (La10-xSi6O27-δ, LSO) have been synthesized by various methods, the effect of the second phase and the conductive mechanism have been discussed. Based on the foundation of alternate current impedance technology, the relationship, capacitance of grain or grain boundary and total conductivity, has been simulated by Zview software. Besides, the effects of grains and grain boundaries on conductivities have been discussed when the grain size is less than 100 nm. The work includes the following aspects.(1) YSZ and La10-xSi6O27-δhave been synthesized by coprecipitation method, and the structural effect on conduction has been invetigated, too. The concentration of charge carrier and the mechanism of ionic transportation change with the structure change from tetragonal to cubic types in YSZ materials were studied. The effect of La content in the apatite La10-xSi6O27-δon the channel of oxygen migration and material conductivity were approached.(2) Composites have been prepared with nano-powders of YSZ and LSO, and the conductive mechanism has been discussed by combining space charge theory, interface strain field and percolation theory. In spite of heavily doped electrolytes, the width of space charge region is very narrow, so the space charge effect could be neglected. However, the misfit dislocations at interfaces are the key factors for the ionic migration. Percolation theory implies that the connectivity of the higher conductive interface determines the total conductivities of composites.(3) YSZ-15wt%LSO nanocomposite has been synthesized by a modified coprecipitate method. The mechanism of surface modification and the related effect have been discussed. As the modified agents, polyethylene glycol dispersed the precipitated particles by space steric effect, but citric acid worked by electrostatic repulsion. The sintering driving force originates from the higher activity and surface energy of nanopowders, and the volume diffusion is the key for the densification of ceramics.(4) The conductivity of LSO containing 10wt%YSZ has been studied. The conductivity of the composite depends on the strain and defects formed at the interfaces. Combining the XRD analysis and interfacial conductive theory, the conductive mechanism has been discussed. The conduction models changed between grain effect and grain-boundary effect at various temperatures and measurement frequencies were investigated by analyzing the AC impedance spectra. It concluded that the interfacial conductivity determines the total conductive property of the compiste electrolyte.