Quenching Effects in Lead-Free Bismuth Based Ferroelectric Ceramics

Abstract

Environmental issues surrounding the use of toxic substances such as lead in electronic products have led to intensive research efforts to replace piezoelectric lead zirconate titanate (PZT) ceramics with alternative lead-free materials. Consequently, bismuth-based solid solutions having comparable ferroelectric and piezoelectric properties with those of PZT ceramics have been identified, including bismuth ferrite (BiFeO3, BF)- and sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT)-based materials. The current project is focused on Na0.5Bi0.5TiO3- NaNbO3 (NBT-NN) and BiFeO3- SrTiO3 (BF-ST) solid solutions as candidate materials for applications in pulsed-power dielectric energy storage devices, electromechanical actuators and high temperature piezoelectrics, respectively. However, issues such as the low depolarization temperature of NBT-NN ceramics and high leakage current / weakening of ferroelectric ordering in BF-ST ceramics present significant barriers to commercial exploitation. Quenching from high temperatures has been introduced to alleviate the degree of these disadvantages in other NBT- and BF- based ceramics, inspiring the current research to establish the connection between quenching-induced structural and electrical property changes and investigate the underlying mechanisms. NBT ceramics modified with 5 and 10 mol% sodium niobate (NBT-5NN and NBT-10NN) were prepared by conventional solid reaction and sintering, followed by quenching from high temperatures (> 900 °C). Quenching induced improved ferroelectric ordering, leading to enhanced lattice distortion and increased depolarisation temperatures. It was also observed that the quenched samples exhibited higher electrical conductivity, due to a higher concentration of oxygen vacancies resulting from reduction at high temperatures. Further reoxidation via annealing at 550 °C recovered the bulk conductivity close to the level of the as-sintered materials but without significant weakening of ferroelectric properties and rhombohedral lattice distortion, indicating that oxygen vacancies are not responsible for the modification of crystal structure and ferroelectric ordering. In situ synchrotron X-ray diffraction measurements for as sintered and quenched NBT-10NN ceramics revealed an electric field-induced irreversible phase transformation from the pseudocubic to rhombohedral structure, confirming the non-ergodic relaxor ferroelectric nature of this composition. Pre-poled samples were found to exhibit a highly textured domain configuration, which remained relatively stable upon further application of the electric field. On the other hand, the intrinsic lattice strain mechanism provided the major contribution to reversible electric field-induced strain in both the as sintered and quenched samples. The improvement of actuation response and piezoelectric properties caused by quenching was attributed to the increased rhombohedral distortion and intrinsic contributions. The final part of the project is concerned with the influence of quenching on BF-ST ceramics with 37.5 and 40 mol % ST content, located near the polar to non-polar morphotropic phase boundary (MPB). Quenching gave rise to reductions in dielectric loss and conductivity, improved polarisation values and a higher depolarization temperature, due to the phase transformation from pseudocubic to rhombohedral symmetry. In situ synchrotron X-ray diffraction demonstrated that the quenched samples exhibit a higher degree of domain texture and lattice strain simultaneously, inducing larger macroscopic strain and better ferroelectric properties. It is also shown that increasing temperature helps to reduce the coercive field and increase the proportion of ferroelectric domains oriented along the electric field direction relative to that at room temperature. In general, for both NBT-NN and BF-ST ceramics, the structure-property relationships have been investigated and evaluated to develop an improved understanding of the mechanisms that may be controlled by quenching. The results obtained during this work provide evidence to demonstrate the potential of Bi-based ferroelectrics as lead-free candidates for future environmentally friendly electronic devices.

Date of Award	29 May 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ge Wang (Co Supervisor) & David Hall (Main Supervisor)