Microstructure and Electrical Properties of Nonstoichiometric 0.94(Na_0.5Bi_0.5+x)TiO₃–0.06BaTiO₃Lead‐Free Ceramics

Abstract

0.94(Na0.5Bi0.5+x)TiO3–0.06BaTiO3 (x = −0.04, 0, 0.02; named NB0.46T‐6BT, NB0.50T‐6BT, NB0.52T‐6BT, respectively) lead‐free piezoelectric ceramics were prepared via the solid‐state reaction method. Effects of Bi3+ nonstoichiometry on microstructure, dielectric, ferroelectric, and piezoelectric properties were studied. All ceramics show typical X‐ray diffraction peaks of ABO3 perovskite structure. The lattice parameters increase with the increase in the Bi3+ content. The electron probe microanalysis demonstrates that the excess Bi2O3 in the starting composition can compensate the Bi2O3 loss induced during sample processing. The size and shape of grains are closely related to the Bi3+ content. For the unpoled NB0.50T‐6BT and NB0.52T‐6BT, there are two dielectric anomalies in the dielectric constant–temperature curves. The unpoled NB0.46T‐6BT shows one dielectric anomaly accompanied by high dielectric constant and dielectric loss at low frequencies. After poling, a new dielectric anomaly appears around depolarization temperature (Td) for all ceramics and the Td values increase with the Bi3+ amount decreasing from excess to deficiency. The diffuse phase transition character was studied via the Curie–Weiss law and modified Curie–Weiss law. The activation energy values obtained via the impedance analysis are 0.69, 1.05, and 1.16 eV for NB0.46T‐6BT, NB0.50T‐6BT and NB0.52T‐6BT, respectively, implying the change in oxygen vacancy concentration in the ceramics. The piezoelectric constant, polarization, and coercive field of the ceramics change with the variation in the Bi3+ content. The Rayleigh analysis suggests that the change in electrical properties of the ceramics with the variation in the Bi3+ amount is related to the effect of oxygen vacancies.

The effect of Fe‐acceptor doping on the electrical properties of Na1/2Bi1/2TiO3 and 0.94 (Na1/2Bi1/2)TiO3–0.06 BaTiO3

“…The phenomenon has been previously attributed to the presence of increased oxygen vacancy concentration in literature. Higher oxygen vacancy concentration is expected to impact the grain boundary migration energy which leads to enhanced grain growth . The insets in Figure C,D represent backscattered electron images of the 4 mol% Fe‐doped compositions with highlighted secondary phases.…”

Dielectric relaxation and electrical conduction in (BixNa1−x)0.94Ba0.06TiO3 ceramics

“…All samples, except x = 0.475, are considerably dense. With a decrease in x , the grain size of the ceramics gradually increases from about 0.5 to 3 μm, which can be attributed to the involvement of defects created by the bismuth volatility and the substitution of Bi 3+ with Na + . The overall reaction can be described using Kröger‐Vink notation,…”

“…It is considered that highly polarizable lone pair electrons of the Bi 3+ play an exceptionally important role in the dielectric and conduction. Excessive Bi 3+ suppressed the frequency dispersion and the conduction current in BNT and BNT‐based ceramics, and led to a pinched hysteresis loop because of the presence of the non‐ferroelectric state with antiparallel atomic shifts between Bi(Na) and Ti cations along the fourfold axis [001] . However, Bi 3+ deficiency enhanced the conductivity .…”

“…However, Bi 3+ deficiency enhanced the conductivity . Previous studies were focused on the electrical properties, including piezoelectric, dielectric, ferroelectric, and the leakage current in Bi‐deficient BNT‐based materials …”

Role of oxygen‐vacancy in piezoelectric properties and fatigue behavior of (Bi0.5Na0.5)0.93Ba0.07Ti1+xO3 ceramics

“…Sung et al reported that the nonstoichiometric quantities of Na and Bi exerted different effects on the macroscopic properties of BNT ceramics, eg, piezoelectric coefficient ( d 33 ) and depolarization temperature ( T d ). It was reported that, in the case of the BNT–0.06BT ceramics, the T d increased with an excess or deficient quantity of Bi 3+ , whereas nonstoichiometric quantities of Ti had an inverse reduction effect on T d . It is well known that during fabrication, the volatilization of Na 2 O and Bi 2 O 3 can produce positively charged oxygen vacancies that are vitally important to the ceramic's piezoelectric properties, due to high carrier mobilities .…”