铌酸钠钾-钛酸盐无限固溶体的电学性能和微区结构的构效关系

K0.5Na0.5NbO3 (KNN)-titanate (ATiO3) solid solutions show diffuse phase transition and high dielectric permittivity/piezoelectric stability over a broad temperature region, as well as a very high field-induced strain, which depends on the characteristic of local structure. However, the characteristic of the fine structure, such as small distortion of tetragonal and octahedral tilting, often is not obvious in XRD patterns. In this proposal, KNN-ATiO3 bi-component and multi-component infinite solid solutions will be investigated in order to focus on the relationship between substitute of ions and local structure, as well as electrical properties. The structure analysis will be done in different scales primarily. Firstly, the dependence of macroscopical phase structure and local structure of the solid solutions on component and temperature will be studied using XRD and Raman scattering technologies, respectively. The difference between macroscopical phase structure and local phase structure, and the influence of phase region on ionic characteristic (such as valence, radium and occupation, etc) and will be revealed. Then, the structure of micro-regions, such as polar/non-polar phase and ferroelectric domain will be detected by the Rietveld refinement and selected area electron diffraction (SAED) experiment. Focusing on the large temperature stability of dielectric permittivity and electrical field induced strain of KNN based ceramics, the intrinsic relationship between phase transition of different phases under the applied electrical field/temperature and physical properties will be discussed. Thirdly, according to structure information and the physical model of dielectric relaxation of the ceramics, the "phase diagrams" of structure and electrical properties will be shown. Finally, considering the phase diagrams, we focus on the characteristic of micro-region structure to use multiphase composite method to develop new lead-free ceramics with giant field-induced stain and high dielectric temperature stability.

K0.5Na0.5NbO3(KNN)与多数钛酸盐(ATiO3)的固溶体显示了高介电/压电温度稳定性、超宽的弥散相变和极高的电致应变。优异的电学性能与特殊的微区结构密切相关，但四方结构的微小畸变和氧八面体倾斜等精细结构信息在XRD图谱上不明显。为此，本项目以KNN-ATiO3无限固溶体为研究对象，首先从结构入手，在不同尺度上分析相结构。采用XRD和Raman等确定宏观和局部相结构随组分和温度的演化，揭示宏观和局部相结构的差异以及离子类型对相区扩展/收缩的影响机制；通过结构精修和SAED确定固溶体中极性/非极性相、电畴等微区的精细结构，针对超宽弥散相变和电致应变，探讨电场/温度作用下不同相之间的转化行为与物理性能之间的内在联系；结合结构参数和介电弛豫物理模型，获得KNN基固溶体的结构和电学性能的“相图”。在此基础上，以微区结构为主线，通过多相复合方法设计大电致应变和高介电温度稳定性的无铅材料。

近些年来，电子材料的无铅化取得了很大的进展。铌酸钠钾（KNN）和钛酸铋钠（BNT）是两种最有竞争力的组分，有望部分取代当前使用的锆钛酸铅（PZT）组分。本项目以KNN-ATiO3（A=Sr, Ba, Bi0.5Na0.5等）为研究对象，研究块体材料的宏观相结构和局部结构对称性随成分和温度的变化规律，重点关注两相共存区的位置、相分布和介电温度行为，获得KNN-ATiO3无限固溶体的结构相图；从离子价态和半径、第二组元结构、晶格畸变等角度揭示相形成和演化的机理；利用介电弛豫模型和结构变化参数对扩散相变进行表征；对照结构相图，绘制电学性能与成分的关系图。结合相图，以微区结构为主线，采用相复合方法（极性相/非极性相，反铁电相-弛豫体相等）构建多元KNN基化合物，开发高介电温度稳定性、高电致应变的无铅材料。.基于同步辐射、中子衍射、透射电镜衍射结果，明确KNN-BNT的固溶度由能量平衡决定，与离子电荷（静电能）和离子半径（弹性能）有关，获得该固溶体精细的结构相图和物理性能相图。采用变温XRD和模谱研究了Bi0.5Na0.5TiO3- SrTiO3- K0.5Na0.5NbO3的PNR弛豫时间的热演化过程。介电温谱包含4个区域3个峰，分别对应三方PNR的弛豫行为，三方-四方转变引起的扩散相变，以及四方PNR的弛豫行为，首次将BNT基材料的介电温谱的微观过程解析清楚。提出了一个唯像统计模型，对扩散转变和弛豫体的Tm之上和之下的介电常数实现了全谱拟合，无论其是对称的还是不对称的。用该模型还可以获得局部电荷载流子（极化子）和导电荷电载流子的组合效应，明确在宽温度范围内不同频率下介电常数的贡献机制。通过改变陶瓷的晶体结构和缺陷载流子输运行为来对其介电性能进行调控，从而获得高且稳定介电常数、低介电损耗的HTCC材料。