提高三临界无铅压电陶瓷温度稳定性的实验研究与理论模拟

The Pb-based piezoelectrics like lead zirconate titanate (PZT) have been the workhorse of piezoelectric devices for half a century. However, Pb has recently been expelled from many applications owing to its toxicity. Thus there is an urgent demand for high performance Pb-free alternatives for the Pb-based piezoelectrics. Recent literatures and our previous studies show that a tricritical-point-type phase boundary in Pb-free systems can lead to equally excellent piezoelectricity as soft PZT at room temperature, owing to a nearly vanishing polarization anisotropy. However, the current tricritical-point-type phase boundaries are highly tilted in the temperature-composition phase diagram of BaTiO3-based systems and also possess fairly low Curie temperatures. Consequently the piezoelectric properties exhibit high temperature sensitivity, largely restricting their applications.In the present project, we proposed an idea to tune the slope of the tricritical-point-type phase boundary by adjusting the phase stability of rhombohedral and tetragonal terminals in the phase diagram through defect doping. It is expected that when the rhombohedral and tetragonal terminals equally compete there will be a vertical phase boundary at which piezoelectric properties show high temperature stability. The project is organized as follows. Firstly, investigations will be done on modeled BaTiO3 based systems to check the feasibility and generality of the above idea. Secondly, after establishing a Landau model for systems with a tricritical-point-type phase boundary, Ginzburg-Landau simulation will be employed to clarify the key aspects that determine the slope of the phase boundary. Combining the experiments and simulation results, a guideline for developing a vertical tricritical-point-type phase boundary will be proposed. Finally, following the guideline, a prototype of (K0.5Na0.5)NbO3 based Pb-free piezoelectric system with vertical tricritical-point-type phase boundary will be developed. Our study not only tries to understand the mechanism which determines the slope of tricritical-point-type phase boundary, but also provides a potential new method for developing new Pb-free alternatives with high temperature stability.

目前使用的大多数压电材料都含有对人体及环境有害的铅元素，因此急需实现压电材料的无铅化。文献和申请人前期研究表明，在无铅体系中构建一条始于三临界点的相界，可以在这种相界处获得较高的压电性能。然而，该相界在温度成分相图上较倾斜，且其居里温度较低，导致这种三临界无铅压电陶瓷材料性能的温度稳定性较差，难以满足实用要求。本申请项目拟通过调整三临界相界两侧四方相和三方相的相稳定性，使两相自发极化及自发应变趋于一致，以实现该相界的垂直化，达到提高其性能温度稳定性的目的。拟首先通过实验在钛酸钡基压电陶瓷体系中验证这一思路的可行性与普遍性；进而以实验结合朗道理论模拟的方法，研究相界两侧铁电相稳定性对相界附近相失稳过程的影响，提出设计高温度稳定性三临界无铅压电陶瓷的准则；最后在高居里温度铌酸钾钠体系中应用上述准则，构建垂直三临界相界，为开发较宽温度范围内性能稳定的无铅压电陶瓷材料作有益探索。

目前使用的大多数压电材料都含有对人体及环境有害的铅元素，因此急需实现压电材料的无铅化。文献和申请人前期研究表明，在无铅体系中构建一条始于三临界点的相界，可以在这种相界处获得较高的压电性能。然而，该相界在温度成分相图上较倾斜，且其居里温度较低，导致这种三临界无铅压电陶瓷材料性能的温度稳定性较差，难以满足实用要求。.本项目通过搜寻决定三临界相界斜率的主要影响因素，建立了预测相界斜率的统计模型，实现了相界的垂直化，达到了提高其性能温度稳定性的目的；并以实验结合理论模拟的方法研究了三临界相界附近相失稳过程的，为开发较宽温度范围内性能稳定的无铅压电陶瓷材料提供了帮助。.通过本项目的研究，主要取得了以下成果和结论：.（1）设计并绘制了15个不同BaTiO3基无铅压电陶瓷材料相图，验证了通过调整三临界相界两侧铁电相相稳定性使相界垂直化这一思路的可行性与普遍性；.（2）利用机器学习算法，揭示了决定三临界相界斜率的两个关键物理因素，即，相图两端材料的晶胞体积比与离子位移比；.（3）以实验结果结合Landau 理论模型以及贝叶斯算法，提出了一套构建垂直三临界相界的设计准则；.（4）建立准同型相界唯像WLR晶体学模型，证明了材料中四方相和三方相的比例变化使极化方向发生转动，提出控制三方相和四方相的比例有可能是优化其性能温度稳定性的一条有效途径。.本项目在国际SCI期刊上共发表论文13篇，其中包括PNAS 1篇，Nat Comm 1篇，Acta Mater 2篇，Sci Reps 1篇，Appl Phys Lett 1篇，PSSB 2篇，MSEA 1篇，J Appl. Phys. 2篇，J Phys. Cond. Mater. 1篇，Ceram. Intern. 1篇。项目组成员共参加国际会议3次，并作口头报告。本项目的研究探明了决定三临界相界斜率的关键因素，提出了构建垂直三临界相界的设计准则，为开发新型无铅压电材料提供了一定的思路和理论指导。