Lead-free BiFeO3-BaTiO3 high temperature piezoceramics

Abstract

High temperature piezoelectrics are a crucial material group that are used in actuation and sensing applications working in harsh environments. However, due to the limited material selection range providing cost-effective, high temperature reliability and lead-free content, the research in this area has been accelerated to find a potential candidate conforming these properties. For this purpose, the BiFeO3-BaTiO3 solid solution system has been proposed as a potential candidate and this study aims to investigate its (micro)structure-property relationships in detail. In the present study, the selected compositions are prepared via the solid state reaction method. It is found that without modification, the ceramics show high conductivity. Therefore, systematic doping strategies are employed using MnO2, La2O3 and TiO2. Nonetheless, during these investigations, a serious issue of chemical heterogeneity is identified, which has substantial impact on the functional properties. Such heterogeneity is detected in the form of core and shell regions which are BiFeO3-rich and-depleted, respectively. The formation mechanism for such core-shell-type microstructures is discussed in terms of kinetic factors and thermodynamic immiscibility, which are found to be directly linked to the incorporation of MnO2 and the influence of donor substitution of La3+ and Ti4+ ions on the solubility of the perovskite end-members (BiFeO3 and BaTiO3). Their influence on the phase content, microstructure and functional properties are significant and give rise to unique features such as discontinuity in ferroic domains across the grain morphology, prominent pseudocubic phase content, suppressed polarisation, reduced electrostrain, and temperature-dependent dielectric anomalies. On the other hand, it is shown that the application of thermal quenching treatment induces substantial alterations in the crystal structure of the shell phase, which transforms from pseudocubic symmetry (short-range ferroelectric ordering) to untilted rhombohedral symmetry (long-range ferroelectric ordering). Such a transformation is confirmed using high resolution and in-situ high energy X-ray synchrotron diffraction investigations. The in-situ studies are carried out on the samples under electric field and reveal a novel actuation mechanism which is induced by quenching. The effect of quenching on polarisation, temperature-dependent dielectric properties and magnetoelectric coupling is also investigated, by comparing with the slow-cooled states of the studied compositions. It is also found that the most dramatic alterations in structure and properties, due to quenching, are evident in the compositions with high BiFeO3 content (≥75%).

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Cernik (Supervisor) & David Hall (Supervisor)