掺杂A2B2O7型陶瓷的有序无序转变与离子传输特性关联研究

Doped A2B2O7-type zirconate ceramics, which exhibit either ordered pyrochlore- or disordered defective fluorite-type structures depending upon the ionic radius ratio, are potential candidates for applications as intermediate-temperature solid electrolyte materials in SOFCs. There exists an important influences of oxygen vacancy concentration, order-to-disorder transformation and phase interfacial structure on the thermal expansion behavior and electrical properties. The doped rare-earth zirconates show the highest conductivity when there is an optimum carrier density for the relatively low ordered pyrochlore-type phase at the vicinity of the phase boundary between fluorite- and pyrochlore-type phases. Therefore, it is of great significance for us to understand the relationship between the order-to-disorder transformation and the oxide-ion transport properties in the intermediate temperature range. This project provides a new approach to investigate the design and fabrication of doped A2B2O7-type ceramics at different oxygen partial pressure, the calculation of oxygen vacancy concentration, and reveal the order-to-disorder transformation by co-doping metallic cations with different valances and rare-earth cations with different ionic-radius ratios to partially substitute the A or B site cations in A2B2O7 ceramics. This project focuses on the crystal structure and interfacial characteristics of novel rare-earth zirconates, oxide-ion diffusion kinetics process and electrical conductivity to reveal the relationship between structural transformation and oxygen vacancy migration, especially the mechanisms associated with the short-range disordering in the pyrochlore-structure and the short-range ordering in the defective fluorite-type structure. The envisaged proposal will be able to identify the relationship among the oxygen vacancy concentration, order-to-disorder transformation, thermal expansion and oxide-ion transport properties, and to further provide both experimental and theoretical support for practical applications of high-performance A2B2O7-type zirconate oxide-ion conductors in the intermediate-temperature range.

掺杂A2B2O7型陶瓷是潜在的中温固体电解质材料，同时具有有序烧绿石型和无序缺陷型萤石相结构，氧空位缺陷浓度、有序无序转变、相界面结构等对其热膨胀性能和电学性能有重要的影响，电导率的最大值在短程无序的烧绿石相内，因此对有序无序转变行为与离子传输特性之间的内在关系进行研究具有重要的科学意义。本项目拟从掺杂A2B2O7型陶瓷材料的设计、氧空位浓度的计算和有序无序转变研究入手，通过不同氧分压气氛烧结及A、B位不同价态阳离子掺杂等手段制备不同类型的阳离子掺杂A2B2O7陶瓷，在研究其物相及界面结构、热膨胀性能和电学性能的基础上，阐明烧绿石型A2B2O7陶瓷中短程无序化转变和缺陷萤石型A2B2O7陶瓷中的短程有序化转变及其机制，揭示氧空位浓度、有序无序转变与热膨胀性能及离子传输性能之间的内在联系，为高性能掺杂A2B2O7型陶瓷在中温燃料电池方面的应用提供理论指导。

掺杂A2Zr2O7陶瓷是具有广泛应用前景的中温固体电解质材料，具有有序烧绿石型和无序缺陷萤石型结构，氧空位缺陷浓度、有序无序转变、相界面结构对其电学性能和热膨胀性能等有重要影响。因此，通过掺杂改性、改变氧空位浓度可调控A2Zr2O7陶瓷的离子传输特性，研究有序无序转变行为与离子传输特性之间的内在关系具有重要的科学意义。本项目从掺杂A2Zr2O7陶瓷的设计、氧空位浓度的计算和有序无序转变研究入手，通过不同氧分压气氛烧结及A、Zr位不同价态阳离子掺杂等手段制备不同类型的阳离子掺杂A2Zr2O7陶瓷，在研究其物相及界面结构、热膨胀性能和电学性能的基础上，揭示氧空位浓度、有序无序转变与热膨胀性能及离子传输性能之间的内在联系，为高性能掺杂A2Zr2O7陶瓷在中温燃料电池方面的应用提供理论指导。根据Brouwer近似和电中性条件，对A位和Zr位掺杂不同价态元素后的氧空位浓度随氧分压变化的关系进行计算设计，发现随氧分压降低所得材料的氧空位浓度增大。异价阳离子掺杂引起的缺陷化学反应对氧空位浓度有直接影响。添加合适的烧结助剂如NiO、ZnO或 Fe2O3等，可大幅提高A2Zr2O7陶瓷的烧结致密化过程，降低烧结温度200oC，同时保持高的氧离子导电性。添加1wt.% NiO、ZnO烧结助剂，GdSmZr2O7陶瓷仍为烧绿石结构；而Fe2O3与GdSmZr2O7陶瓷发生反应生成少量第二相Gd0.5Sm0.5FeO3。综合采用X射线衍射、高分辨电镜、选区电子衍射和拉曼光谱等手段，揭示了A2Zr2O7陶瓷烧绿石相的短程无序化转变和缺陷萤石相的短程有序化转变与机制。掺杂A2Zr2O7陶瓷的平均线膨胀系数与8YSZ相比略有升高。掺杂A2Zr2O7陶瓷的晶粒电导率与单胞自由体积和结构的有序化程度有关。在位于烧绿石相与缺陷型萤石相的边界附近有序度较低的烧绿石相的晶粒电导率最大。真空烧结部分有序(Sm0.5Gd0.5)1.95Yb0.05Zr2O7陶瓷在1173K时电导率为1.16×10–2S•cm–1。通过对比不同氧分压下的电导率或者氧浓差电池阐明了掺杂A2Zr2O7陶瓷的导电机理，发现在不同氧分压测试条件下A位掺杂A2Zr2O7陶瓷的总电导率不随氧分压的变化而变化，仍为纯氧离子导体。计算发现Zr位掺杂Nb使氧空位减少，但由于引入质子电导，1173K下电导率提高至1.41×10–2S•cm–1。