Quenching enhanced lattice polarizability in lead-free ferroelectrics

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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.