Quenching enhanced lattice polarizability in lead-free ferroelectrics

压电材料约占世界电陶瓷市场的25%，由于其小型化的潜力，预计将进一步增长。虽然这些应用大多使用钙钛矿Pb(Zr,Ti)O3 (PZT)，但由于对其毒性和环境破坏的担忧不断升级，人们正在采取措施用无铅替代品取代它。在过去的二十年里，对无铅铁电体的广泛研究已经得出了一个普遍的共识，即没有任何一类无铅材料可以取代用途广泛的PZT。在逐步淘汰铅基铁电材料的过程中，无铅材料有望取代PZT，首先是在要求相对较低的高功率超声波应用中。在几种无铅材料中，铋基材料具有显著的优势。与PZT相比，（1-x）Na1/2Bi1/2TiO3-xBaTiO3 （NBT-BT）组合物具有非常好的高功率特性，因此是有益的。BiFeO3 （BF）与Bi3+表现出类似于Pb2+的孤对效应，是另一种有望取代PZT用于高温应用的材料。然而，这些材料必须经过微调，以适应设备操作下的特定条件。最近的研究表明，淬火可以提高铋基铁电体的热稳定性和铁电性能。在本项目中，将研究淬火铋基材料的作用，以(a)优化淬火条件，以避免微裂纹并获得所需的材料性能，(b)了解晶格畸变增强的结构根源，这归因于热稳定性的提高和铁电性能的改善，以及(c)建立淬火作为一种替代加工路线，以定制功能材料的材料性能。这将通过研究具有代表性的弛豫剂NBT-BT和铁电性BF-BT成分来实现。本文讨论的初步工作为优化淬火条件提供了依据，并结合电学和力学性能测量确定了加工路线。平均（同步衍射）和局部（对分布函数分析和透射电镜）结构分析将探讨阳离子有序、Bi3+偏离中心和淬火引起相移的细节。在松弛器的特殊情况下，将从极性纳米区域的详细表征和量化来研究淬火诱导的铁电有序。本研究建立的结构-微观结构-加工-性能的综合相关性有望促进淬火作为一种通用工具来定制功能材料的性能。

Piezoelectric materials constitute about 25% of the world’s market of electroceramics and is expected to grow further due to their potential for miniaturization. While majority of these applications employ the perovskite Pb(Zr,Ti)O3 (PZT), steps are being taken to replace it with lead-free alternatives due to escalating concerns of toxicity and environmental damage. Extensive research on lead-free ferroelectrics in the past two decades has resulted in a general consensus that, there is no single class of lead-free material that can replace the versatile PZT. In the phasing out of lead-based ferroelectrics, the lead-free materials are foreseen to replace PZT, firstly in high power ultrasonic applications that demand relatively lower stringent requirements. Of the several lead-free classes of materials, bismuth (Bi) based materials offer significant advantages. The (1-x)Na1/2Bi1/2TiO3-xBaTiO3 (NBT-BT) compositions are beneficial due to their exceptionally good high power characteristics, in comparison to PZT. BiFeO3 (BF) with Bi3+ exhibiting lone-pair effect similar to Pb2+, is another promising material foreseen to replace PZT for high temperature applications, in future. Yet, these materials have to be fine-tuned to suit specific conditions under device operation.It was recently demonstrated that quenching enhances the thermal stability and ferroelectric properties of Bi based ferroelectrics. In this project, the role of quenching Bi-based materials will be investigated to (a) optimize the quenching conditions to evade microcracking and obtain desired material properties, (b) understand the structural origin of the enhanced lattice distortion, which is attributed to the increased thermal stability and improved ferroelectric properties and (c) establish quenching as an alternate processing route to tailor material properties of functional materials. This will be achieved by studying the representative relaxor NBT-BT and ferroelectric BF-BT compositions. The preliminary work discussed in the proposal provides the basis for optimizing the quenching conditions, which combined with electrical and mechanical property measurements will establish the processing routes. The average (synchrotron diffraction) and local (pair-distribution function analysis and transmission electron microscopy) structural analysis will probe the details of cation ordering, Bi3+ off-centering and quenching induced phase shifts. In the specific case of relaxors, quenching induced ferroelectric order will be investigated from the detailed characterization and quantification of polar nano regions. The comprehensive structure-microstructure-processing-property correlation established from this study is expected to promote quenching as a generic tool to tailor properties of functional materials.