局部结构相变和缺陷随机场对再入型弛豫铁电体形成的影响研究

Reentrant relaxor is a kind of materials which can transform from a relatively more ordered state(ferroelectric state) to a less ordered state(relaxor state) with temperature decreasing. This phase transition violates the traditional law that materials always transform from a less ordered state to a more ordered one with cooling and it is thus a peculiar transformation.Also reentrant relaxor always accompanies with high energy storage density with high energy efficiency. Therefore, unveiling the mechanism of the formation of reentrant relaxor has both scientific and technological significance. However, due to the lack of quantitative or semi-quantitative theoretical analysis the mechanism of the formation of reentrant relaxor is still unclear. Recently our preliminary experiments found that at the morphotropic phase boundary of BaZr0.35Ti0.65O3-xPbTiO3 system, reentrant relaxor transition occurs(Xiaoqin Ke,Xiaobing Ren, et. al., to be published). This finding tells that reentrant relxor is greatly associated with local structural transformation as well as different effects of local fields caused by defects on different phases. We plan to use phase field modeling combined with experiments to study the influence of two important factors- local structural transformation and defect random fields, on the formation of reentrant relaxor as well as its energy storage properties. This study can unveil the mechanism of the formation of reentrant relaxor and guide the designing of new ferroelectric materials with high energy storage density and high efficiency.

再入型弛豫铁电体是一种随着降温发生从相对有序相(正常铁电体)到相对无序相(弛豫铁电体)转变的材料，该相变与材料随着降温通常从无序到有序的转变规律相悖，因此是一种比较特殊的相变，并且该相变通常还伴随着高效率的高储能密度等优异性能，因此揭示其形成机理具有较重大的理论意义和应用前景。但是由于缺少定量或半定量的理论分析目前对其形成机理还不是很清楚。最近我们的前期研究发现，BaZr0.35Ti0.65O3-xPbTiO3体系的准同型相界附近出现了再入型弛豫铁电体（柯小琴，任晓兵等，待发表)，预示了再入型弛豫铁电体的形成与局部结构相变以及缺陷随机场对不同相的不同作用密切相关。本课题拟通过相场模拟并结合实验手段，研究局部结构相变和缺陷随机场两大因素对再入型弛豫铁电体形成及储能性能的影响。该研究能够揭示再入型弛豫铁电体的形成机理并能指导设计新型高效率的高储能密度铁电材料。

再进入弛豫铁电体及普通弛豫铁电体等特殊的铁电体材料由于具有大电致伸缩应变、高储能密度、大压电系数等优异性能而在近些年来备受关注。理解再进入弛豫铁电体及普通弛豫铁电体的形成机理对于根据实际需求设计这些特殊铁电材料具有重要的意义，然而缺陷掺杂是如何导致普通弛豫铁电体及再进入弛豫铁电体的形成还不是很清楚。本项目从缺陷掺杂可能引起的三种局部效应（局部场效应、局部相变温度效应、局部相变类型效应）出发，具体研究了缺陷的三种局部效应对于再进入弛豫铁电体及普通弛豫铁电体形成的影响。研究发现，缺陷的局部场效应及局部相变温度效应分别是普通弛豫铁电体特征温度TB及T*形成的物理原因，而两种效应的共同作用使得普通弛豫铁电体同时具有三个特征温度（TB、T*、Tf）。在此基础上，我们进一步发现要想形成再进入弛豫铁电体，缺陷掺杂还需引入额外的局部相变类型效应，即掺杂一定浓度的缺陷后需要能改变铁电相变的类型在体系相图中形成准同型相界。该局部相变类型效应可以使得材料在发生正常铁电相变后在局部形成新的纳米铁电相从而出现再进入弛豫铁电体现象。在以上理论研究的基础上，我们设计了一个新型弛豫铁电体系，该体系将弛豫铁电体与准同型相界耦合在一起从而得到准同型相界弛豫铁电体，该准同型相界弛豫铁电体同时具备弛豫铁电体的低滞后特性及准同型相界的大应变特性，在高精度控制器等设备上具有潜在的应用价值。我们的工作揭示了再进入弛豫铁电体及普通弛豫铁电体形成的物理机制，对于设计高性能弛豫铁电体材料具有指导意义。