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 (Wrec = 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.

Re-entrant relaxor ferroelectric behaviour in Nb-doped BiFeO3–BaTiO3 ceramics

“…It was observed that Ba and Ti were depleted in the core regions, while the signals corresponding to Bi and Fe were slightly enhanced, similar to previous observations in donor-doped BF–BT based ceramics. 18,38 It was also shown previously that the BF-rich core regions exhibited high rhombohedral distortion and contained well-developed ferroelectric domain configurations in comparison with the BT-rich, relaxor-like pseudo-cubic shell regions. 39…”

“…It was observed that Ba and Ti were depleted in the core regions, while the signals corresponding to Bi and Fe were slightly enhanced, similar to previous observations in donor-doped BF-BT based ceramics. 18,38 It was also shown previously that the BF-rich core regions exhibited high rhombohedral distortion and contained well-developed ferroelectric domain configurations in comparison with the BT-rich, relaxor-like pseudo-cubic shell regions. 39 Comparing undoped and Nb-doped 0.7BF-0.3BT ceramics, it was observed previously that single phase materials could be obtained for Nb substitution levels less than 1 at%, whereas secondary phases corresponding to Bi 5 Ti 3 FeO 15 and BaNb 2 O 6 were identified for dopant concentrations of greater than or equal to 1 at% Nb.…”

“…The grain size was also progressively reduced with the increase in Nb concentration. 18 On the other hand, Nb appeared to be distributed evenly across the examined area, although the validity of the results may be questionable in view of the limited sensitivity of the EDS detector for low dopant concentrations. There was little evidence of well-ordered ferroelectric domain configurations, such as those found in rhombohedral and tetragonal PZT ceramics, 13 in the polished cross-sections, although irregular features with relatively light contrast were present in the images in back-scattered electron (BSE) mode.…”

“…39 Comparing undoped and Nb-doped 0.7BF-0.3BT ceramics, it was observed previously that single phase materials could be obtained for Nb substitution levels less than 1 at%, whereas secondary phases corresponding to Bi 5 Ti 3 FeO 15 and BaNb 2 O 6 were identified for dopant concentrations of greater than or equal to 1 at% Nb. 18 In comparison with single component BiFeO 3 , where small changes in stoichiometry or impurity levels commonly lead to second phase formation, 40 it is evident that the presence of BaTiO 3 may help to stabilise the perovskite structure against decomposition in the presence of aliovalent dopants and/or impurities. The grain size was also progressively reduced with the increase in Nb concentration.…”

“…As noted above, the resistivity can be improved by substituting donortype Nb 5+ cations for Fe 3+ or Ti 4+ , but high Nb concentrations tend to suppress long-range ferroelectric ordering and induce ergodic relaxor ferroelectric behaviour. 17,18 The corresponding 'softening' effect of Nb 5+ doping, with the expected enhancement of ferroelectric and piezoelectric properties, was not generally observed and may have been overlooked in studies that employed relatively high Nb concentrations 41 at%.…”