Enhancement of Nonlinear Dielectric Properties in BiFeO3-BaTiO3 Ceramics by Nb-Doping

Abstract

BiFeO₃-BaTiO₃ (BF-BT) ceramics exhibit great potential for 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 work, the influence of pentavalent niobium substitution for trivalent ferric ions on the structure, microstructure and dielectric properties of 0.7BF-0.3BT ceramics was investigated systematically. Doping with niobium led to incremental reductions in grain size (from 7.0 to 1.3 µm) and suppression of long-range ferroelectric ordering. 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, at ~470 and 600 °C, which were 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 (W_rec = 2.01 J cm^-3) and energy storage efficiency (η = 68%) of the 0.7BiFeO₃-0.3BaTiO₃ binary system were achieved at 75 °C under an electric field of 15 kV mm^-1. This material demonstrates the greatest potential for applications in energy storage capacitors and temperature-stable dielectrics.

Keywords: bismuth ferrite–barium titanate; core-shell microstructure; energy storage capacitors; ferroelectric; lead-free piezoelectric; niobium doping; non-linear dielectrics; relaxor ferroelectric.

Figure 1

(a) Room temperature full X-ray diffraction patterns, (b) {111} peak profiles and (c) {200} peak profiles for the 0.7BF–0.3BT–xNb ceramics (0 to 5 at% Nb). Arrow labels in (b,c) indicate peaks associated with Cu Kα₂ X-rays.

Figure 2

Scanning electron microscope images (back-scattered mode) and histograms of grain size distributions for 0.7BF–0.3BT–xNb ceramics with (a) x = 0, (b) x = 0.005, (c) x = 0.01, (d) x = 0.02, (e) x = 0.03 and (f) x = 0.04.

Figure 3

Back-scattered (BSE) SEM image and elemental mapping of the 0.7BF–0.3BT–4Nb ceramic. The red rectangular area defines the region used for micro-chemical mapping.

Figure 4

Temperature-dependence of relative dielectric permittivity (ɛ_r) and loss (tan δ) for (a) 0.7BF–0.3BT–0Nb (b) 0.7BF–0.3BT–2Nb and (c) 0.7BF–0.3BT–xNb ceramics, measured at 10 kHz. Dashed lines in (a,b) indicate dielectric loss values.

Figure 5

(a) Polarization–electric field (P–E) loops of 0.7BF–0.3BT–xNb ceramics, (b) maximum and remanent polarization (P_m and P_r) and coercive electric field (E_C) as a function of Nb concentration, (c) recoverable energy density (W_rec) and efficiency (η) as a function of Nb concentration, measured at room temperature.

Figure 6

Polarization–electric field (P–E) loops of 0.7BF–0.3BT–2Nb ceramics under different electric fields at (a) 25 °C and (d) 75 °C. Maximum and remanent polarization (P_m and P_r) and difference (P_m − P_r) as a function of electric field for 0.7BF–0.3BT–2Nb at (b) 25 °C and (e) 75 °C. Recoverable energy density (W_rec) and energy storage efficiency (η) as a function of electric field of 0.7BF–0.3BT–2Nb at (c) 25 °C and (f) 75 °C.

Figure 7

Schematic diagram illustrating the reorientation of PNRs under the influence of an applied electric field in the 0.7BF–0.3BT–2Nb ceramic, with both ergodic (reversible) and non-ergodic (partially irreversible) relaxor ferroelectric characteristics.