基于晶界偏析调控的CeO2基电解质微结构工程及相关机理研究

Doped ceria has been regarded as one of the most promising candidate electrolytes for intermediate-temperature (IT) and low-temperature (LT) solid oxide fuel cells. The grain-boundary resistivity dominates the overall resistivity of doped ceria at low and intermediate temperatures. The barrier-layer effect caused by impurity precipitation and the space-charge effect caused by dopant segregation are the main factors which lead highly resistive grain-boundaries; hence the segregation and scavenging of the grain-boundary dopant/impurities has always been one of the important issues in the field of ceria-based electrolytes. Recently, applicant found that the addition of ZnO can promote the reaction between the segregated dopant and the impurity SiO2, which may “co-scavenging” two kinds of barrier effect. Based on the control of grain-boundary dopant/impurities segregation via “co-scavenging” reaction, this project focuses on the relationship between the ion transmission capacity and the component/property of grain-boundary, aimed to reveal the converting mechanisms about the wetting/dewetting configurations of grain-boundary phases and illustrate the principles of grain-boundary dopant segregation; these research will establish a theoretical basis for the precise control of the distributions of grain-boundary segregated phases, and will provide reliable theories for the fabrication of high-performance ceria-based electrolytes.

掺杂CeO2被认为是中低温SOFC电解质材料的重要候选之一。处于低温及中温区间的CeO2基电解质，材料电阻主要来源于晶界。由溶质偏析所引发的晶界空间电荷效应和由杂质汇聚所导致的晶界电荷阻挡效应是晶界电阻的两大主要来源，因此晶界溶质/杂质的偏析与清扫一直都是CeO2基电解质领域研究的重要课题之一。申请人在最近的研究工作中发现添加ZnO可有效地促进偏析的溶质和杂质SiO2发生反应，借助此反应将有望实现两种晶界电阻效应的“共清扫”。本课题拟在利用此类“共清扫”反应调控晶界溶质及杂质偏析的基础上，深入研究晶界微观结构组成特性与离子传输能力间的联系，以进一步揭示晶界相浸润/非浸润（wetting/dewetting）特性转变机制，阐明晶界溶质偏析规律，为实现晶界偏析相分布形态的精确控制奠定理论基础，并为高性能CeO2基电解质的制备提供可靠的理论指导。

晶界的结构与特性强烈影响着氧化铈基电解质材料的宏观电性能。由溶质偏析所引发的晶界空间电荷效应和由杂质汇聚所导致的晶界电荷阻挡效应是氧化铈基电解质晶界电阻的两大主要来源。本项目通过晶界微结构工程，阐明了晶界杂相的偏析与汇聚规律，揭示了决定晶界偏析相浸润/非浸润（wetting/dewetting）分布形态的关键机理，并首次实现了两种晶界电阻效应的“共清扫”。主要研究结果如下：（1）在烧结过程中，具有浸润特性的无定型富硅杂质相极易沿晶界传播，引发晶界电导的剧烈恶化。（2）对无定型杂质相形成高效清扫的关键点是使其析晶，从浸润的玻璃态转化为非浸润的晶态。（3）CaO可引起无定型的杂质相以Ca2xRE8x(SiO4)6O2x/2（RE稀土溶质，如Gd、Sm等）的形式强烈析晶，形成对晶界中偏析溶质与汇聚杂质的“共清扫”。（4）在烧结过程中，非均匀性分布的CaO必须借助ZnO、Co2O3等助剂所引发的液相环境才能高效地完成“共清扫”反应。（5）反应剂（CaO）+促进剂（ZnO）的协同策略被成功地应用于提高氧化铈电解质的SiO2杂质容忍度，可使Ce0.8Sm0.2O1.9 (SDC)电解质耐受10000 ppm以下的SiO2污染而晶界电性能没有明显的衰退。（6）在准确调控反应剂和促进剂含量的基础上，通过精确控制体系中的SiO2含量，可以同时提升电解质的晶粒与晶界电性能，此时SiO2的角色转变为有利于电性能的添加剂；经优化后的具有微米级晶粒的电解质在低温段晶界电阻仅为晶粒电阻的1/5~1/10，总电导率在500 ºC附近即可达到10-2 S/cm。本项目的研究不仅为氧化铈基电解质材料的微结构设计与优化奠定理论与技术基础，对其他受限于晶界溶质偏析或杂质汇聚的功能类陶瓷也具有重要的借鉴意义。