钙钛矿型铁电材料中一种负压增强途径的理论与实验探索

Stress and strain can significantly influence the structure and properties of ferroelectrics. Unlike the ferroelectric thin films clamped on the substrates, ferroelectric single crystals and ceramics work in a freestanding state and thus cannot benefit from the tuning by stress or strain. Creating a beneficial stress state in ferroelectric bulks while keeping their outer boundaries free is a challenging but meaningful subject. Based on the literature research, we propose a route in which the negative pressure is created by the surface tension of dense close pores and can possibly enhance the properties of ferroelectric bulks. The proposed route will be investigated in detail by both theory and experiments. The following studies will be performed: Constructing the phenomenological theory to check the generality of the enhancing role of the negative pressure in perovskite type ferroelectrics, carrying out finite element simulation on the stress field due to the dense close pores, examining the existence of the pore-induced negative pressure effect in single porous PbTiO3 whiskers. The program is devoted to supply important clues on realizing the stress-tuning of the properties of ferroelectric single crystals and ceramics.

应力应变对于铁电材料的结构和性能有显著的调控作用。然而与受基底夹持的铁电薄膜相比，铁电单晶和陶瓷在使用条件下外边界处于自由状态，因而限制了利用应力应变对其性能进行增强的可能性。如何在保持外边界自由的前提下在铁电块材中引入有利于其性能的应力状态，是一个挑战性很强但具有重要价值的课题。本项目在文献调研的基础上提出一种利用大量孔的表面张力在内界面上对实体部分的拉伸作用来构造负压效应进而对铁电块材进行性能增益的思路，并将对该负压增强途径从理论和实验上进行详细探索。具体研究内容包括: 构建唯象理论考察负压增强效应在钙钛矿型铁电体中的适用性；对密集闭合微孔在实体部分中产生的应力场进行有限元模拟；在多孔PbTiO3晶须中验证微孔化导致负压效应的可行性。本项目的实施可为实现铁电单晶及陶瓷性能的应力调控提供重要的参考。

本项目在文献调研的基础上提出一种利用负压效应进而对机械自由状态的铁电体进行性能增益的思路，并对该负压增强途径从理论和实验上进行了详细探索。研究发现由较低密度前驱体转化而来的钙钛矿型PbTiO3晶须在一定直径范围内其晶格自发畸变存在异常增大，同时其自发极化强度及居里温度也有明显增益。定量分析表明这些增强结果与理论所预测的负压所导致的性能增强效应高度吻合，对应的负压可达GPa量级。对晶型转换过程的相场模拟进一步揭示出在钙钛矿型PbTiO3晶须中观测到的负压增强效应正是源于其前驱体向钙钛矿转变过程中由表及里、逐步收缩的转化机制。通过构建普适唯象理论及第一性原理研究，我们发现具有一阶顺电-铁电相变的PbTiO3、BaTiO3及Pb(Zr,Ti)O3几个钙钛矿型铁电体在负压下的其铁电及压电性能均有明显增益。