Quenching effects in BiFeO3-BaTiO3 ceramics

Abstract

Lead free piezoelectric ceramics have attracted great attention in the past two decades due to growing environmental concerns. In particular, BiFeO3-BaTiO3 (BF-BT) ceramics are considered as one of the most promising candidate materials for applications in high temperature devices such as ultrasonic transducers, sensors, capacitors. In the present work, the processing-structure-microstructure-property relationships of BF-BT ceramics are evaluated with reference to the influence of high temperature annealing and quenching procedures. Manganese-doped BF-BT ceramics were synthesised using conventional solid state reaction and then annealed at a range of different temperatures followed by air quenching. The crystal structure transformed from pseudo-cubic to rhombohedral, accompanied by increasing rhombohedral distortion as a function of the quenching temperature; this results in a transformation from relaxor to normal ferroelectric behaviour as well as inducing improvements in the Curie temperature, depolarization temperature and piezoelectric properties. Meanwhile, the domain structure evolved gradually from disordered polar nano regions (PNRs) into larger lamellar domains with the increasing quenching temperature, which was found to be consistent with crystal structure and functional properties. In addition, the structure of the near-surface region was investigated using grazing incidence X-ray diffraction (GIXRD), which revealed the presence of a distorted surface structure induced by grinding processes; it is proposed that the presence of this distorted near-surface region may be the origin of the so-called tetragonal phase in BF-BT ceramics, reported previously by some authors. The occurrence of micro-chemical homogeneity in BF-BT ceramics, dependent on the powder milling procedures and subsequent quenching effects, is also investigated. It is demonstrated that the use of high energy milling can yield improved homogeneity and enhancement of various ferroelectric/piezoelectric properties. Furthermore, the time-dependent relaxation of the piezoelectric charge coefficient, d33, after DC poling is evaluated to provide evidence of the instability of the remnant polarisation of the as-sintered sample, in comparison with that of the quenched sample, which can be correlated with the corresponding modification of crystal structure. In addition, solely inter-granular fracture behaviour is observed in the as-sintered sample, while the quenching process modifies it into a mixed inter- and intra-granular mode due to changes in the morphology of the grain boundary phase. Microstructural examination of the ferroelectric domain configurations in as-sintered, annealed and quenched BF-BT ceramics revealed a systematic evolution from predominantly disordered nano-scale domains (virgin sample), to mixed nano-domains and ordered lamellar domains (annealed sample), and well-ordered nano-scale herringbone type domain patterns (quenched sample). Differences in the non-180˚ domain switching characteristics of these materials were quantified using in-situ electric field-dependent synchrotron XRD and correlated with the corresponding macroscopic polarisation-electric field hysteresis loops. High temperature XRD was also used to demonstrate the structural transformation from rhombohedral to cubic at the Curie point (400 to 500 ˚C, dependent on sample type) and an anomalous reduction of the rhombohedral distortion in each of the samples on cooling from 200 to 30 ˚C, which is attributed to a form of re-entrant relaxor ferroelectric behaviour.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Cernik (Supervisor) & David Hall (Supervisor)