Sodium Bismuth Titanate Research Articles

Erratum to: Effect of Hf and Li on the structure and electrical properties of Bi0.5Na0.5TiO3 lead-free ceramics

Lead-free (Bi0.5Na0.5−y Li y )(Ti1−x Hf x )O3 piezoelectric ceramics were prepared using a conventional solid-state method, and the effects of Li and Hf on phase structure and electrical properties were systematically investigated. All the ceramics exhibited a rhombohedral phase, which is independent on the doped Li and Hf elements and contents. With the increase of Hf4+ or Li+, their T d values gradually decreased. Their piezoelectric and dielectric properties can be effectively enhanced by optimizing the amount of Hf4+ and Li+, and maximum d 33 value of ~100 pC/N and e r value of ~850 were achieved for x = 0.03, y = 0.1 and x = 0.12, y = 0, respectively. In addition, it was found that the ferroelectric and strain behavior was gradually weakened with the addition of Li and Hf. This research helps understand the influence of Li and Hf on the structure and properties of bismuth sodium titanate materials.

Recoverable Self-Polarization in Lead-Free Bismuth Sodium Titanate Piezoelectric Thin Films.

Bismuth sodium titanate, Bi0.5Na0.5TiO3 (BNT), is a promising lead-free ferroelectric material. However, its potential applications have not been fully explored, mainly because of the complex domain structure arising from its intricate phase transitions. A deep and thorough study of its domain structure and polarization switching behavior will greatly help with understanding the polarization nature and with promoting future applications. In this work, we demonstrate that BNT polycrystalline films possess a highly ordered out-of-plane polarization (self-polarization) and randomly oriented in-plane polarizations. Interestingly, the inherent nature of polarization in the BNT films does not allow for the nonvolatile domain writing, as the switched polarization spontaneously and rapidly reverses to the initial orientation state once the external poling electric field is removed, making the self-polarization recoverable. Such a stable self-polarization vanishes gradually with temperature increasing over 150 °C but starts to recover to its initial state upon cooling down to 250 °C, and entirely recovers once the temperature is reduced to below 200 °C. Such interesting properties of BNT films are attributed to the combined effects of the free charges at the Pt electrode, (detected) cation vacancies at the oxide/Pt interface and the defects in oxide lattices. Our results make a step closer to fully understand the nature of polarization and related piezoelectricity in BNT. Such films with recoverable self-polarization are of great interest for applications as sensors, actuators, and transducers that can operate particularly under high temperatures and high electric field conditions.

Composition‐induced phase transitions and enhanced electrical properties in bismuth sodium titanate ceramics

AbstractHere, the composition‐induced phase transition and enhanced electrical properties were investigated in terms of lead‐free {[Bi0.5(Na0.82−xK0.18Lix)0.5]1−ySry}TiO3 (BNKLST‐x/y, x=0‐0.175 and y=0‐0.125) ceramics. The rhombohedral and tetragonal phase boundary can be constructed, and then the enhancement of piezoelectric properties (d33~271 pC/N and kp~0.38) can be observed for x=0.10 and y=0.05, which is superior to most reported results in polycrystalline BNT‐based ceramics. In particular, a fatigue‐free behavior after 106 polarization switching cycles was shown in the BNKLST‐0.10/0.05 ceramics due to the reversible field‐induced phase transition, and a slight decrease in d33 (~4.5%) was also shown. More importantly, the general model of electric field, temperature, and composition‐induced phase transition was employed to explain the enhancement of piezoelectric and fatigue properties. We believe that the composition design of this system can promote the development of bismuth sodium titanate lead‐free ceramics.

A correlative study on strain and variation of coercive field in lead-free (Na0.5Bi0.5)TiO3–Bi(Mg0.5Zr0.5)O3–Bi(Mg0.5Ti0.5)O3 ternary system

Sodium bismuth titanate–bismuth magnesium zirconate–bismuth magnesium titanate (NBT–BMZ–BMT) lead-free ternary ceramics are prepared by solid-state reaction method. Consequence on structural, dielectric, ferroelectric properties and piezoelectric coefficient is investigated. Single phase rhombohedral R3c phase is retained in all poled samples. Coercive field decreases due to minimum homogeneous strain (δ) which facilitates domain reorientation and domain switching. Reduced homogeneous strain is the key factor which lowers the coercive field and enhances the dielectric constant. NBT–BMZ–BMT ternary ceramics reduces the coercive field and possesses high remnant polarization, dielectric constant and piezoelectric coefficient—the important requirement in functional devices.

Stress-induced phase transition in lead-free relaxor ferroelectric composites

Piezoelectric materials are considered an enabling technology generating an annual turnover of about 20 billion $. At present, lead-based materials dominate the market with the known risk to health and environment. One of the three key competitors for their replacement is the class of sodium bismuth titanate (NBT)-based relaxor ferroelectrics, the use of which is limited by thermal depolarization. An increased thermal stability has recently been experimentally demonstrated for composites of Na1/2Bi1/2TiO3-6BaTiO3 with ZnO inclusions (NBT-6BT:xZnO). However, the exact mechanism for this enhancement still remains to be clarified. In this study, piezoresponse force microscopy and 23Na NMR spectroscopy were used to demonstrate that the incorporation of ZnO leads to a stabilization of the induced ferroelectric state at room temperature. Temperature-dependent measurements of the relative dielectric permittivity ε′(T), the piezoelectric coefficient d33 and the strain response revealed an increase of the working temperature by 37 °C. A simple mechanics model suggests that thermal deviatoric stresses stabilize the ferroelectric phase and increase, as well as broaden, the temperature range of depolarization. Our results reveal a generally applicable mechanism of enhancing phase stability in relaxor ferroelectric materials, which is also valid for phase diagrams of other ceramic matrix composites.

Growth, mechanical and domain structure studies of Na0.5Bi0.5TiO3 single crystal grown by flux growth method

Single crystal of sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) has been grown by a high-temperature flux growth method. The mechanical properties of polished single crystal plates were investigated. The Vickers microhardness, (Hv) number of the NBT crystals was found to be ∼3. The fracture toughness and brittleness index of the single crystal were calculated from R-curve is 0.15166MNm−3/2 and 276 m−1/2, respectively. Domain structure and birefringence of <001>pc cut NBT single crystal have been analyzed using polarized light microscopy. At room temperature, a stripe-like dark and bright domain pattern have been observed with preferred orientation along <110> direction having birefringence value of 2 × 10−3. With increasing temperature, birefringence was found to decrease with the disappearance of stripe-like pattern near 225 °C. However, with further increase in temperature, the stripe-like pattern re-appears above 240 °C with polarization reversal. On cooling similar behavior was observed near 205 °C. These results are in concurrence with the dielectric/piezoelectric measurements, and unusual behavior of domains was attributed to the ferroelectric→ferroelastic transition in NBT.

Controllable synthesis and growth mechanism of lead free bismuth sodium titanate nanowires

Abstract Highly uniform lead-free piezoelectric bismuth sodium titanate (Bi 0.5 Na 0.5 TiO 3 , BNT) nanowires were successfully synthesized via a hydrothermal method. Synthesized at a stirring speed range of 300–1000 rpm, the compositions and orientations of BNT nanowires were well controlled. The effects of stirring speeds on the formation of BNT nanoparticles and nanowires in the hydrothermal processes were systematically investigated. The BNT nanowires with a high aspect ratio were proven to be single crystals with [110] growth direction from high-resolution TEM analysis. The mechanism of growth of BNT nanowires out of nanoparticles in the hydrothermal processes was proposed.

Bismuth sodium titanate lead‐free piezoelectric coatings by thermal spray process

AbstractBismuth sodium titanate (BNT) piezoelectric ceramic coatings with dense morphology and single perovskite phase were fabricated by thermal spray process. The melting and crystallization behaviors of BNT near congruent composition and the effect of excess Bi composition were investigated by analyzing the heating‐melting‐cooling cycle for the application in thermal spray process. Crystal structure and morphology of the thermal sprayed BNT coatings were examined, and their electrical and electromechanical properties were tested. An effective piezoelectric coefficient d33 of 50 pm/V was obtained under substrate clamping, measured with laser scanning method. The findings have shed light on the design of appropriate compositions for achieving high quality lead‐free piezoelectric ceramic coatings by the scalable thermal spray process.

Sodium bismuth titanate-based lead-free RAINBOW piezoelectric devices

The piezoelectric properties of lead-free piezoelectric ceramics are normally lower than those of lead oxide-based ceramics. In order to enhance the electromechanical performance of lead-free piezoelectric ceramics, an asymmetric chemical reduction was applied to sodium bismuth titanate (NBT)-based piezoelectric ceramics. Similar to the lead-containing ceramics, a curvature structure can be induced by the reduction in the NBT-based materials and lead-free RAINBOW (reduced and internally biased oxide wafer) devices can be fabricated. A large displacement (approximately 17μm) under an electric field of 900V/mm and high piezoelectric sensitivity (>4000pC/N) under a stress, which are related to the reduction induced curvature, can be measured in the NBT-based devices. The apparent piezoelectric response of the lead-free RAINBOW devices is comparable to that of Pb(Zr,Ti)O3-based devices. We proposed that apart from the piezoelectric properties, flexoelectric effect could also be a contributing mechanism for the observed apparent piezoelectric response in RAINBOW devices.

Formation of sodium bismuth titanate—barium titanate during solid‐state synthesis

AbstractPhase formation of sodium bismuth titanate (Na0.5Bi0.5TiO3 or NBT) and its solid solution with barium titanate (BaTiO3 or BT) during the calcination process is studied using in situ high‐temperature diffraction. The reactant powders were mixed and heated to 1000°C, while X‐ray diffraction patterns were recorded continuously. Phase evolutions from starting materials to final perovskite products are observed, and different transient phases are identified. The formation mechanism of NBT and NBT–xBT perovskite structures is discussed, and a reaction sequence is suggested based on the observations. The in situ study leads to a new processing approach, which is the use of nano‐TiO2, and gives insights to the particle size effect for solid‐state synthesis products. It was found that the use of nano‐TiO2 as reactant powder accelerates the synthesis process, decreases the formation of transient phases, and helps to obtain phase‐pure products using a lower thermal budget.

The preparation of lead-free bismuth sodium titanate ceramics via the solid state combustion technique

ABSTRACTLow calcined temperature, high density and good dielectric properties of BNT ceramics successfully prepared by the solid state combustion technique and glycine was used as fuel to reduce the reaction temperature. The effects of calcination temperature (500–900°C) and sintering temperature (1100–1150°C) on phase formation, microstructure and dielectric properties of (Bi0.5Na0.5)TiO3 (BNT) ceramics were investigated. The phase purity, crystal structure and microstructure of the samples were tested using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the pure rhombohedral perovskite structure of BNT powders were observed from the sample calcined at 650°C, which is lower than the solid state reaction technique by ∼200°C. The purity phase was detected in all ceramic samples. The microstructure of the BNT powders exhibited an almost-spherical morphology and a highly agglomerated form. The average grain size of the ceramics was increased with increase sintering temperature. The TF-R and Tm were slightly increased with the increase of sintering temperature. The highest density and the best dielectric properties was obtained by sample sintered at temperature of 1120°C. The solid state combustion technique is a new alternative for reduction of calcined temperature, improvement of densification and modify of dielectric properties to prepared BNT ceramics.

Simulation and Experimental Data of P-E Hysteresis Loop in BNT and BKT

Hysteresis loop measurements performed both experimentally for Bismuth Sodium Titanate (BNT) and Bismuth Potassium Titanate (BKT) samples using modified Sawyer-Tower Circuit. The experiment showed that the P-E hysteresis had main parameters for BNT, remnant polarization (Pr = 27 μC/cm2), spontaneous polarization (Ps = 35 μC/cm2) and coercive electric field (Ec = 60 kV/cm) and for BKT the remnant polarization was (Pr = 5.2 μC/cm2), spontaneous polarization (Ps = 30 μC/cm2) and coercive electric field (Ec = 4.72 kV/cm). These three parameters (remnant polarization, spontaneous polarization, coercive electric field) were used in a simulated software depending on the mathematical model for the polarization in ferroelectric materials. The simulation software predicted the value of applied electric field required to perform the P-E hysteresis experiment varying with Pr, Ps, Ec. The results of the simulation exhibited agreement with the experimental data. The last prediction could help the researchers in studying the ferroelectric hysteresis loop, especially for those studying a fatigue behaviour or studying the effect of electric field cycle on the hysteresis loop.

Ionic conductivity of acceptor doped sodium bismuth titanate: influence of dopants, phase transitions and defect associates

The temperature dependent ionic conductivity of NBT results from an interplay of defect complex formation, phase coexistence, and dopant concentration.

B-site substituted solid solutions on the base of sodium-bismuth titanate

The paper presents results of studies of the formation of phases during the solid-state synthesis in the [([Formula: see text] system of solid solutions with [Formula: see text]-site substitutions. The substitutions by zirconium, tin and ion complexes (In[Formula: see text]Nb[Formula: see text] and (Fe[Formula: see text]Nb[Formula: see text] have been studied. It has been found that the synthesis is a multi-step process associated with the formation of a number of intermediate phases (depending on the compositions and calcination temperatures). Single-phase solid solutions have been produced at the calcination temperatures in the interval 1000–1100[Formula: see text]C. An increase in the substituting ions concentration leads to a linear increase of the crystal cell size. At the same time, the tolerance factor gets reduced boosting the stability of the antiferroelectric phase as compared to that of the ferroelectric phase.

Study of amorphous and crystalline phases of sodium bismuth titanate thin films by optical and Raman spectroscopy

Amorphous and crystalline Na0.5Bi0.5TiO3 (NBT) thin films were deposited at different substrate temperatures on fused silica by plused laser deposition method. The phase and microstructure of the deposited films were investigated by XRD and FE-SEM respectively. The noticeable difference is observed in the Raman and optical spectra of amorphous and crystalline films. To grasp further insight, the dispersion data of the refractive index is fitted with the single oscillator model. The oscillator strength and oscillator energy are computed. The refractive index of NBT thin films decrease with increase in wavelength and approach a constant value in long wavelength limit while extinction coefficient increase with increase in wavelength. In Raman spectra of amorphous materials, many Raman peaks coalesce and form very broad spectral bands, whereas the polycrystalline material shows distinct band.

Enhanced bulk conductivity of A-site divalent acceptor-doped non-stoichiometric sodium bismuth titanate

Bismuth-deficient sodium bismuth titanate (nominally Na0.5Bi0.49TiO2.985, NB0.49T) is a good oxide-ion conductor. Here we report the influence of A-site divalent ions, M2+=Ca2+, Sr2+ and Ba2+, on the electrical properties of NB0.49T. A-site divalent doping for Bi3+ enhances the bulk (grain) conductivity by ~one order of magnitude without changing the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on the doping mechanism Bi3++½ O2−→M2+. Among these three dopants, Sr2+ is the most effective in increasing the bulk conductivity due to a combination of its smaller mismatch in ion size with Bi3+, its intermediate polarisability and lower bond strength to oxygen compared to Ca2+ and Ba2+. Doping strategies for further improvements to bulk conductivity of NBT materials are discussed based on these results. Comparison with other oxide-ion conductors and initial stability test under reducing atmosphere show the doped non-stoichiometric NBT materials are promising for low and intermediate temperature applications.

ChemInform Abstract: High Ionic Conductivity with Low Degradation in A‐Site Strontium‐Doped Nonstoichiometric Sodium Bismuth Titanate Perovskite.

AbstractPolycrystalline phase‐pure Na0.5Bi0.47Sr0.02TiO2.975 is prepared by calcination of appropriate amounts of Na2CO3, Bi2O3, TiO2, and SrCO3 (800 °C, 2 h) followed by ball milling, pelletizing, and final annealing (1150 °C, 2 h).

Combustion technique synthesis of lead-free piezoelectric bismuth sodium titanate-bismuth potassium titanate–barium titanate ceramics

ABSTRACTFabrication of lead-free piezoelectric ceramic bismuth sodium titanate–bismuth potassium titanate–barium titanate, focusing on the morphotropic phase boundary (MPB) between the rhombohedral and tetragonal phases was studied. a[0.935(Bi0.5Na0.5)TiO3-0.065BaTiO3]-(1-a)[0.8(Bi0.5Na0.5)TiO3-0.2(Bi0.5K0.5)TiO3]; BNBK ceramics with 0 ≤ a ≤ 1 were successfully fabricated using the combustion technique. All BNBK samples were sintered at 1150 °C for 2 h. The effect of a changing on the phase formation, microstructure, dielectric properties and piezoelectric properties of perovskite BNBK ceramics were investigated. XRD results of BNBK ceramics with 0 ≤ a ≤ 1 showed the rhombohedral-tetragonal morphotropic phase boundary (MPB). The SEM results showed the average grain size is between 1.18 and 2.56 µm. The effect of the a content had no influence on the tendency of grain sizes. The maximum density, shrinkage, dielectric constant at Curie temperature (ϵr), remanent polarization (Pr), coercive field (Ec) and piezoelectric constant (d33) were found from BNBK ceramic with a = 1, which is around 5.76 g/cm3, 18.43%, 6503, 15.4 μC/cm2, 20.1 kV/cm and 289 pC/N, respectively.

Variations in the morphotropic phase boundary with Bi deficiency on Ba-substituted sodium bismuth titanate

We successfully demonstrated that a slight Bi deficiency causes the rhombohedral-tetragonal morphotropic phase boundary (MPB) to shift to the considerably lower Ba content in a (Na0.5Bi0.5)1−xBaxTiO3 (Ba-substituted NBT) system. Rietveld analysis revealed that the composition yielding the MPB was located at x = 0.10 when using the polymerizable complex method to prevent evaporation of sodium and bismuth and at x = 0.068 in samples of (Na0.5Bi0.5−z)1−xBaxTiO3−δ with a Bi deficiency of z = 0.03.

Structural and electromechanical properties of Na0.5Bi0.5TiO3 ceramics produced by different synthesis routes

Sodium bismuth titanate, Na0.5Bi0.5TiO3 (NBT) ceramics were produced by three different methods; conventional mixed-oxide (CMO) route, molten salt synthesis (MSS) and topochemical microcrystal conversion (TMC) and then sintered at 1150 oC for 2 h in air atmosphere. The crystal structure, dielectric, ferroelectric and field-induced strain properties were investigated for all samples. All samples showed a single phase perovskite structure without any evidences of unwanted secondary phases. The NBT ceramics synthesized by the TMC method show slightly better dielectric, ferroelectric and field induced strain response as compared with CMO and MSS synthesized ceramics. The room temperature dielectric constant measured at 1 kHz increased from 218 for NBT ceramics synthesized by MSS method to 271 and 330 for CMO and TMC synthesized ceramics, respectively. Similarly, the dynamic piezoelectric coefficient (d33*) enhanced from 91 pm/V for CMO synthesized to 97 pm/V and 107 pm/V for MSS and TMC synthesized ceramics, respectively.