Synthesis and Characterizations of SrBi2Ta2O9 Modified NBT-BT and NBT-KNN Ferroelectric Ceramics near Morphotropic Phase Boundary

Abstract

In the scientific community, ferroelectric ceramics have attracted great attention because of their unique dielectric, piezoelectric, pyroelectric and ferroelectric properties. These properties of ferroelectric ceramics make them suitable for various multifunctional device applications. Till date, owing to excellent dielectric, piezoelectric, pyroelectric and ferroelectric properties, the Pb(Ti,Zr)O3(PZT) based ferroelectric ceramics, near morphotropic phase boundary (MPB), are widely used in applications such as actuators, sensors, etc. However, the toxicity, problems in recycling and disposal are the major environmental concerns against the use of lead-based ferroelectric materials. Therefore, there is an urgent need for searching effective lead-free ferroelectrics, whose properties are comparable to those of the well-known PZT based materials. Among the effective lead-free ferroelectrics, the (Na0.5Bi0.5)TiO3(NBT) system has drawn great attention in recent years. Although, the NBT system shows a strong ferroelectric behavior, yet it has some critical limitations such as (i) high coercive field, (ii) high conductivity, (iii) high dielectric loss and (iv) high leakage current, which goes against the use of this system in various device applications. Efforts have been made to overcome these limitations by doping or synthesizing solid solutions with other systems. The solid solution of the (1-x)NBT-xBT system exhibits an MPB at x≈0.07. Similarly, in the solid solution of the (1-x)NBT-xKNN system there exist an MPB at x≈0.07 between the rhombohedral FE phase and a tetragonal AFE phase. Therefore, it is imperative to study structural, dielectric, piezoelectric, ferroelectric, polarization fatigue and leakage current density properties of the (1-x)NBT-xBT and (1-x)NBT-xKNN systems near their respective MPBs. Selected Perovskite Materials
 (1-x)Na0.5Bi0.5TiO3-xBaTiO3/NBT-xBT
 (1-x)Na0.5Bi0.5TiO3-x K0.5Na0.5NbO3/NBT-xKNN
(Where x=0.05, 0.06, 0.07, 0.08)
Solid solutions of the NBT-xBT and NBT-xKNN (where x=0.05, 0.06, 0.07, 0.08) systems were prepared by solid state reaction route. The XRD study confirmed the single perovskite phase in both the NBT-xBT and NBT-xKNN systems at 1000oC and 800 oC calcination temperatures, respectively. No remarkable change in the grain size (average grain size ~2.5μm) was observed with the variation of BT content in the NBT–xBT system. Whereas, in the NBT–xKNN system, the average grain size first increases with the increase of KNN content up to x=0.07 and then starts decreasing for x=0.08. Highest experimental density was found to be ~5.89g/cc (98.30% of the theoretical density (ρth)) and 5.77g/cc (97.81% of the ρth) for x=0.07 compositions of the NBT-xBT and NBT-xKNN systems, respectively. Dielectric study showed existence of both Td (depolarization temperature) and Tm (temperature corresponding to maximum dielectric constant) in all the systems with diffusive phase transition behavior. Dielectric constant (εr) (at 1 kHz frequency) at room temperature (RT) and at Tm were found to be ~2275 and ~5067, respectively for the x=0.07 ceramic samples of the NBT-xBT system. Whereas, for the x=0.07 ceramic samples of the NBT-xKNN system, εr (at 1 kHz frequency) at RT and at Tm were found to be ~2787 and ~4438, respectively. Leakage current density was found to be between 10-7-10-6A/cm2 and ~10-6A/cm2 for all the NBT-xBT and NBT-xKNN ceramics, respectively. In the NBT-xBT system, optimum values of remnant polarization (Pr) ~ 31.71μC/cm2, piezoelectric coefficient (d33) ~105pC/N, electromechanical coupling coefficient (kp) ~0.21 and maximum induced strain% ~0.45 at RT were obtained in the x=0.07 composition. Whereas, optimum values of Pr ~20.61C/cm2, d33 ~78pC/N, kp ~0.12 and maximum induced strain% ~0.36 were obtained at RT in the x=0.07 composition of the NBT-xKNN system. Bipolar polarization fatigue study confirmed the degradation of all the NBT-xBT and NBT-xKNN ceramics after 107 cycles. Excellent dielectric, piezoelectric and ferroelectric properties confirmed the MPB nature of x=0.07 composition of both the systems. Still, both these systems lack the reliability issues such as polarization fatigue, high coercive field, high On the other hand, bismuth layered structure ferroelectrics (BLSF) such as SrBi2Ta2O9, SrBi2Nb2O9, Bi4Ti3O12, SrBi4Ti4O15, etc., are the natural anti-fatigue materials which can withstand 1012 erase/rewrite operations. A detail structural, dielectric, piezoelectric, ferroelectric, leakage current density and polarization fatigue properties of the effective BLSFs systems were carried out.
Selected BLSF Materials SrBi2Ta2O9/SBT, Sr0.8Bi2.15Ta2O9/SBexT and SrBi2(Ta0.925W0.075)2O9/SBTW 2-layered SBT, SBexT, and SBTW ceramics were synthesized in single phase by solid-state reaction technique. SEM micrographs showed the development of plate-like grains with maximum experimental density ~8.87 g/cc (98 % of the ρth) of the SBexT ceramic samples, sintered at 1200oC/4hr. Enhanced transition temperature (Tc), and better εr and ferroelectric properties were observed in both the SBexT and SBTW ceramic samples compared to the SBT ceramic samples. The dielectric study also showed the sharp transition in both the SBexT and SBTW ceramic samples, whereas a diffused phase transition was observed in the SBT system. From dielectric measurements, highest Tc was obtained in the SBexT system. P-E hysteresis loop study confirmed the ferroelectric nature of all the SBT based ceramic samples. The maximum Pr ~ 8.07μC/cm2 with minimum coercive field (Ec) ~ 15.18kV/cm were obtained in the SBexT ceramic samples. Optimum RT value of d33 ~24pC/N and kp ~ 0.098 were obtained in the SBexT ceramic samples. Reduced leakage current density ~3.14x10-9A/cm2 was obtained in the SBexT ceramic samples. Bipolar polarization fatigue study confirmed the negligible degradation of all the BLSF materials even after 109 cycles. Though, the MPB compositions of the NBT-xBT and NBT-xKNN systems exhibit high dielectric, piezoelectric and ferroelectric properties but they show high leakage current density and large degradation of polarization value after repeated cycles. On the contrary, SBexT ceramic samples showed lower leakage current density and better polarization fatigue resistance after repeated cycles. But, the Pr value of the SBexT samples is lower compared to the MPB compositions of the NBT-xBT and NBT-xKNN systems. Therefore, for retaining comparably higher value of Pr and improving the polarization fatigue resistance, these MPB compositions were further modified by SBexT system.
Selected Perovskite-BLSF Materials
(1-ϕ)(0.93 Na0.5Bi0.5TiO3-0.07 BaTiO3/NBT-BT)-ϕSr0.8Bi2.15Ta2O9
(1-ϕ)(0.93 Na0.5Bi0.5TiO3-0.07 K0.5Na0.5NbO3/NBT-KNN)-ϕ Sr0.8Bi2.15Ta2O9 (Where ϕ= 2, 4, 8, 12, 16 wt. %) (1-ϕ)(NBT-BT)-ϕSBexT and (1-ϕ)(NBT-KNN)- ϕSBexT (where ϕ= 2, 4, 8, 12, 16 wt. %) ceramic composites were prepared by solid state reaction route. XRD studies showed the co-existence of individual phases in both the ceramic composite series. SEM study showed the development of plate-like grains for higher content of SBexT phase in both the composite systems. RT εr and tanδ values decreased and diffuse phase transition nature increased with the increase of SBexT content in both the ceramic composite systems. In both the ceramic composite systems, the leakage current density was found to be ~10-7 to 10-8 A/cm2, which is one to two order less than the NBT-BT and NBT-KNN systems. Maximum induced strain% decreased with the increase of SBexT content in both the ceramic composite systems. P-E hysteresis loops of the SBexT modified NBT-KNN ceramics showed the retention of good Pr values. Polarization fatigue studies showed the improvement of polarization fatigue resistance with the increase of SBexT content in both the ceramic composite systems. Improvement in leakage currents density, fatigue resistance, retention of high εr, Pr, d33 and kp for ϕ ≤ 4wt. % of (1-ϕ) (NBT-KNN)-ϕSBexT ceramic composites suggested its usefulness in capacitor, piezoelectric and NVRAM applications.