Doping Effects in BiFeO3-BaTiO3 Ferroelectric Ceramics

Abstract

BiFeO3-BaTiO3 (BF-BT) ceramics are important multiferroic materials, which are attracting significant attention for potential diverse applications in high temperature piezoelectric transducers, temperature-stable dielectrics and pulsed-power capacitors. Further optimization of functional properties for different types of applications can be achieved by modification of processing parameters or chemical composition. In the present project, the effects of Nb5+ as a donor dopant for Fe3+ and Sr2+ as an acceptor dopant for Bi3+ on the structure, microstructure and dielectric, ferroelectric, and piezoelectric properties of 0.7BiFeO3-0.3BaTiO3 (0.7BF-0.3BT) ceramics are evaluated. In the Nb doped BF-BT ceramics, it was found that core-shell type microstructural features became more prominent as the Nb concentration increased, which were correlated with the formation of distinct peaks in the dielectric permittivity–temperature relationship attributed to the BT-rich shell and BF-rich core regions, respectively. Nb-doping of BF–BT ceramics yielded reduced electronic conductivity and dielectric loss, improved electrical breakdown strength and enhanced dielectric energy storage characteristics. These effects are attributed to the charge compensation of pentavalent Nb donor defects by bismuth vacancies, which suppresses the formation of oxygen vacancies and the associated electron hole conduction mechanism. The relatively high recoverable energy density (Wrec = 2.01 J cm−3) and the energy storage efficiency (η = 68%) were achieved in 2 at% Nb-doped 0.7BF–0.3BT ceramics, which demonstrates the greatest potential for applications in energy storage capacitors and temperature-stable dielectrics. Lower concentration of donor Nb5+ (0.5 at%) substitution for Ti4+ was found to be an effective way to increase electrical resistivity, but also led to the formation of a frequency-dependent relaxation of the dielectric permittivity and associated loss peak were observed over the temperature range from -50 to +150 °C. These effects were correlated with anomalous enhancement of the remanent polarization and structural (rhombohedral) distortion with increasing temperature, indicating the occurrence of a re-entrant relaxor ferroelectric transformation on cooling. The results of this investigation provide new insight into the effects of donor-doping in BF-BT ceramics and a route to design environmentally friendly high-temperature piezoelectric ceramics. For the Sr doped BF-BT ceramics, the use of a post-sintering Ar annealing process was found to be an effective approach to reduce electrical conductivity induced by the presence of electron holes associated with reoxidation during cooling. A low Sr dopant concentration (0.3 at %) yielded enhanced ferroelectric (Pmax ~ 0.37 C m-2, Pr ~ 0.30 C m-2) and piezoelectric (d33 ~ 178, kp ~ 0.27) properties, whereas higher levels led to chemically heterogeneous core-shell structures and secondary phases with an associated decline in performance. The electric field-induced strain of the Sr-doped BF-BT ceramics was investigated using a combination of digital image correlation macroscopic strain measurements and in-situ synchrotron X-ray diffraction. Quantification of the intrinsic (lattice strain) and extrinsic (domain switching) contributions to the electric field induced strain indicated that the intrinsic contribution dominated during the poling process, yielding 69.3% and 30.7% for the intrinsic and extrinsic contributions respectively for 0.70Bi0.997Sr0.003FeO3-0.30BaTiO3 ceramics at an electric field of 6 kV/mm.

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