钇稳定氧化锆—硅酸镧复合氧离子导体的制备、导电性及相关机理

【作者】 刘超峰；

【导师】 李志成；

【作者基本信息】 中南大学 ， 材料物理与化学， 2011， 硕士

【摘要】 纳米复合材料集两种或两种以上材料组元的纳米特性和复合特性于一体,从而在性能上表现出优于或完全不同于单个组元的特性。纳米复合后,两相界面占体积比例高,且由于结构的差异在界面处产生高密度缺陷。而界面缺陷区中,空间电荷的重排以及晶格失配引起的应变势场对材料综合导电性的变化产生重要作用。因此,纳米复合技术成为功能材料领域研究发展的重要手段,该技术为改善氧离子电解质材料的离子导电性提供了新思路。论文结合8mol%氧化钇稳定氧化锆(YSZ)和磷灰石型硅酸镧(La10-xSi6O27-δ,LSO)的结构和物化特性,通过不同的实验方法以及两相含量的变化研究复合电解质导电性的变化并讨论相关机理。在交流阻抗技术的理论基础上,利用Zview软件模拟晶粒和晶界电阻电容值变化时材料总电导的变化规律。另外,还重点讨论了晶粒纳米化对材料导电性的影响、纳米尺寸下晶界特性的转变及其产生的效应。工作主要涵盖以下内容：(1)利用共沉淀法合成YSZ和La10-xSi6O27-δ,对比并讨论结构变化对材料导电性的影响。着重讨论立方和四方YSZ结构转变方式以及转变前后载流子浓度及导电通道的变化。分析了La含量变化时磷灰石结构的变化及其如何影响氧离子通道,进而影响材料导电性等问题。(2)通过固相混合的方式将YSZ和LSO纳米粉体复合制备成两相共存的氧离子复合电解质。通过LSO含量的变化,结合空间电荷理论、界面应变场理论和渗流理论讨论两相复合后材料的导电机理。分析认为对于高缺陷浓度的氧离子导体,复合界面区的空间电荷层非常窄,对材料综合导电性的影响可忽略。而界面应变区内的应变和位错群则是氧离子迁移的关键通道并主导界面导电,结合渗流理论说明导电通道的连通性对材料导电性的影响。(3)利用改性共沉淀法制备了YSZ-15wt%LSO纳米复合氧离子导体。主要分析讨论沉淀体表面进行改性处理的机制以及产生的效应。聚乙二醇的大分子结构通过空间位阻效应将沉淀粒子分隔防止团聚,而柠檬酸则是通过静电排斥作用使颗粒分散。另外,纳米颗粒的高活性和高表面能为陶瓷烧结提高了驱动力,体积扩散是材料收缩致密化的关键因素。(4)制备并研究了LSO基体中含有10wt%YSZ时材料导电性的变化。结合X射线衍射结果和相关界面导电理论,分析了复合引起的晶格畸变以及界面区缺陷浓度对导电性的影响通过阻抗谱分析,讨论了不同温度及测试频率下晶粒和晶界导电方式的转变。认为在LSO-10wt%YSZ复合电解质中,晶界导电性质的转变和提高对材料整体导电性变化产生决定性作用。更多还原

【Abstract】 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.更多还原