Na0.5Bi0.5TiO3-BaTiO3 Research Articles

Design and Performance Enhancement of a Piezoelectric Micromachined Ultrasonic Transducer Based on NBBT Lead-Free Piezoelectric Single-Crystal Thin Film

Piezoelectric micromachined ultrasonic transducers (PMUTs) have attracted widespread attention due to their high performance, miniaturization, and easy integration with semiconductor processes. In this paper, a PMUT device based on high-performance and lead-free Na0.5Bi0.5TiO3-BaTiO3 (NBBT) piezoelectric single-crystal thin films was designed for the application of medical high-frequency ultrasonics. Three-dimensional modeling and analysis of PMUT elements on the proposed structure were performed via the finite element method. The relationship between structure configuration in terms of the top electrode and the cavity shape of the bottom was studied. The PMUT properties and its device performance, including resonant frequency, effective electromechanical coupling factor (keff2), and the static sensitivity of different device structures, were analyzed. In addition, by rotating the Euler Angle γ of the NBBT piezoelectric single-crystal film, the static sensitivity and keff2 of the model are improved to 1.34 when γ is rotated to 45 ± 5°. It was shown that the PMUT using rotated NBBT demonstrated an enhanced relative pulse-echo sensitivity of −46 dB and a bandwidth of 35% when the reflective surface was 200 μm. We conclude that the PMUT in accordance with an NBBT piezoelectric single-crystal film designed by simulation offers a high frequency, larger keff2, and high sensitivity, which provides application prospects in high-resolution and high-frequency medical ultrasonic imaging.

Ferroelectric–Ferroelastic Transitions in (Na0.5Bi0.5)TiO3-BaTiO3 Single Crystals

The comprehensive understanding of (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lattice structure is highly desired to develop lead-free ferroelectric materials. However, most of the previous studies focused on the improvement of piezoelectric properties at room temperature, and many structural puzzles are left unclear. In this work, the lattice structure of a ferroelastic phase and the ferroelectric-ferroelastic transitions in both rhombohedral NBT and tetragonal NBT-8%BT single crystals are investigated in detail. Our results illustrate the complex process of the ferroelectric-ferroelastic transition of NBT. The variation of Ti-O modes and oxygen octahedra modes clearly indicates the gradual change of lattice symmetry from R3c to P4bm during a wide temperature range between 170 and 350 °C. A ferroelectric-ferroelastic transition is also confirmed in tetragonal NBT-8BT for the first time, and the lattice symmetry of P4bm is found to be maintained during the ferroelastic stage. This work reveals the lattice evolutions of the ferroelectric-ferroelastic transition of NBT-BT crystals and provides new insights for understanding the ferroelasticity and the evolution of phonon modes in a lead-free relaxor.

Shape memory effect in Na0.5Bi0.5TiO3-based ferroelectric ceramics

Shape memory materials (SMMs) are a class of smart materials that generate recoverable residual deformation. Among the family of SMMs, shape memory ceramics (SMC) are more applicable to extreme situations with high temperature, high stress, or corrosive environments. Most of the well-known SMCs are zirconia-based ceramics. Herein, we demonstrate the shape memory behavior of ferroelectric ceramics through a case study on Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) ceramics. The results show that after heat treatment under stress, there was a large residual strain left in the NBT-BT ceramics. After they were annealed at a temperature above the phase-transition temperature, the residual strain recovered. The maximum recoverable residual strain observed in the NBT ceramic plate was approximately 0.369%. Because most ferroelectric ceramics have a smaller critical stress regarding the stress-induced structural orientation or transformation compared with that of the zirconia-based SMCs, they should be more suitable for SMC applications at a medium stress level.

Origin of enhanced depolarization temperature in quenched Na0.5Bi0.5TiO3-BaTiO3 ceramics

Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT)-based lead-free piezoelectric ceramics have been actively studied in recent years as a potential replacement for lead-based materials in ultrasonic applications. However, its relatively low thermal depolarization temperature (Td) is still an imperative obstacle hindering implementation in practical application. Recently, it was reported that quenching is an effective way to improve Td of NBT-based ceramics, but the essential mechanism is still unclear. In this study, 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (NBT-6BT) ceramics were quenched in air and liquid nitrogen, and then annealed in oxygen and nitrogen atmospheres to explore the origin of enhanced depolarization temperature. The results from this study correlate the enhancement of Td to the residual stress, which induces a stable rhombohedral ferroelectric phase, thereby increasing the thermal depolarization temperature of NBT-6BT. Our results indicate that the residual stress is also an important factor influencing the electrical properties of quenched piezoelectric ceramics, which should be given more attention in future studies.

Theoretical analysis of high electromechanical coupling surface acoustic wave propagating on lead-free Na0.5Bi0.5TiO3–BaTiO3 single crystal

The propagation properties of surface acoustic wave (SAW) on the Na0.5Bi0.5TiO3–BaTiO3 (NBBT) substrate with different crystal cuts were theoretically analyzed by Finite Element Method (FEM). The calculated results, for the NBBT with the optimized electrode type and thickness, indicate the existence of Rayleigh and Sezawa modes with high electromechanical coupling factor K2 ~18% and ~6.8% respectively, which are considerably higher than that of the conventional lithium niobate (LiNbO3) single crystal for SAW applications. The research provides a comprehensive understanding of the SAW propagation characteristics of lead-free NBBT, and explores the promising applications of NBBT in wideband SAW devices.

Random lattice strain and its relaxation towards the morphotropic phase boundary of Na0.5Bi0.5TiO3 -based piezoelectrics: Impact on the structural and ferroelectric properties

We demonstrate that the lead-free piezoelectric compound Na0.5Bi0.5TiO3 (NBT) exhibits random lattice strain in the ferroelectric phase, and that this feature primarily dictates the way the system evolves towards the morphotropic phase boundary in the unpoled state of NBT-based piezoelectrics. Investigations on two different morphotropic phase boundary (MPB) systems, namely Na0.5Bi0.5TiO3 - K0.5Bi0.5TiO3 (NBT-KBT) and Na0.5Bi0.5TiO3 - BaTiO3 (NBT-BT), revealed that the coupled structural-polar evolution towards the MPB is primarily driven by the necessity to minimize this strain. Our study suggests that the random lattice strain originates in the random stacking of the in-phase tilt and antiphase octahedral tilted regions, and that the system is able to minimize it by adopting a sequential stacking of the two tilt types, leading to a long-period modulation in the octahedral tilt configuration over large parts of the sample volume. This hinders the development of long-range ferroelectric order as the MPB is approached. We also demonstrate that the composition showing the maximum piezoelectric coefficient corresponds to a structural state wherein considerable polar-structural disorder coexists with the field-stabilized long-range rhombohedral ferroelectric order after poling, and not coexistence of two ferroelectric phases (tetragonal and rhombohedral), generally believed.

The surface mechanism for the flexoelectric response in sodium bismuth titanate‐based ferroelectric ceramics

AbstractThe flexoelectric effect of the ferroelectric materials has been intensively studied in recent years. One unresolved issue in this field is that the measured flexoelectric coefficients in ferroelectrics are often several orders of magnitude greater than the values predicted by the theoretical studies. To understand the mechanism for this discrepancy, the flexoelectric response of the Na0.5Bi0.5TiO3–BaTiO3 (NBBT) ceramics of different compositions was systematically studied. We show that the measured flexoelectric response of the NBBT ceramics is mainly from the piezoelectric response of the spontaneously polarized surface in the ceramics. The surface effect can be reduced or removed by abrading the surface off or modifying the compositions of the surface. The thickness of the surface layer was found to be 10‐15 µm. We also show that the composition dependence of the flexoelectric response is mainly affected by the piezoelectric response of the surface layer, whereas the temperature dependence of the flexoelectric response of a composition is primarily influenced by the dielectric constant of the bulk. The mechanisms for the observations are discussed based on the origin of the surface layer. This work is important for the understanding of the mechanisms of the flexoelectric (or flexoelectric‐like) response in ferroelectrics.

An investigation into group 13 (Al, Ga, In) substituted (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lead-free piezoelectrics

Ternary phase diagrams were explored for the systems (Na0.5Bi0.5)TiO3-BaTiO3-Bi(M)O3 M = Al, Ga, In to determine periodic trends as a function of substitution. The ferroelectric and piezoelectric properties were measured and compared to (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT). X-ray synchrotron and neutron diffraction data were collected to determine the changes in structure as a function of substitution. Data showed that most systems behave in a similar manner to the parent (Na0.5Bi0.5)TiO3-BaTiO3. However, two specific compositions display interesting properties: the 2% BiGaO3 (BG2) substituted sample and the 4% BiAlO3 (BA4) substituted sample. The BG2 sample undergoes an irreversible electric-field induced phase transition, as seen in NBT-BT, but has a significantly larger high-field strain. The BA4 composition has a different strain mechanism than all the other composition studied. It displays incipient piezoelectricity which results in a giant high-field strain due to a reversible phase transition. This study highlights the difficulty in trying to improve the properties of the highly disordered NBT-BT material.

Elaboration of lead-free Na0.5Bi0.5TiO3–BaTiO3 (NBT-BT) thick films by aerosol deposition method (ADM)

Thick and dense ceramic films of lead-free 0.94Na0.5Bi0.5TiO3-0.06 BaTiO3 (NBT-BT) composition were elaborated by aerosol deposition method (ADM) at room temperature. A powder of suitable grain size was elaborated by solid state reaction. Using this powder, two samples were elaborated by ADM respectively on glass and metallic substrates, in order to check for microstructure and electrical properties. This process allowed obtaining a thick film (3.2µm) with dense microstructure. Measurement of electrical properties revealed a lossy dielectric behavior indicating interfacial phenomena at the electrode–film interface. The measurement of the ferroelectric hysteresis cycle does not show any characteristics of a ferroelectric behavior, but corresponds well to the one of a lossy non-linear dielectric. The absence of ferroelectricity is probably due to the low grain size of the obtained thick film (130nm). Further experiments are in progress in order to try to obtain ferroelectric properties.

The Growth and Properties of Lead-Free Ferroelectric Single Crystals

Much attention is drawn to the preparation, structure and properties investigation of lead-free ferroelectrics for the next generation of piezoelectric devices. (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lead-free solid solution piezoelectric single crystals with composition x in the range of 0–0.05 as a materials with high piezoelectric properties were successfully grown from platinum crucible by using the top-seeded solution growth (TSSG) method. The dimensions of NBT-BT crystal is Ø40 × 10 mm2. X-ray powder diffraction patterns reveal that the crystal structure of NBT-BT crystal changes from rhombohedral to tetragonal symmetry with increasing amounts of BT(x). The dielectric, ferroelectric and piezoelectric properties of NBT-BT crystals with different compositions near the morphotropic phase boundary (MPB) were studied systematically. Ions (Mn, Eu, Zn) doped NBT and NBT-BT 95/5 crystals were also grown and studied. In addition, their piezoelectric and ferroelectric properties are investigated. Further, a high-quality and large-sized (K0.25Na0.75)NbO3 (KNN25/75) single crystal has been achieved by a carefully controlled TSSG method. The dimensions of the as-grown KNN25/75 single crystal reached up to Ø30 × 10 mm2. The obtained KNN crystals provided us a superb material for the dielectric, piezoelectric, ferroelectric and electromechanical coupling property characterization along different orientations.

(Na, Bi)TiO3 based lead-free ferroelectric thin films on Si substrate for pyroelectric infrared sensors

In this study, we report ferroelectric and pyroelectric properties of (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) thin films on Si substrates using chemical solution deposition for the first time. The NBT-BT thin films deposited on Pt/Ti/SiO2/Si substrates have exhibited a typical hysteresis loop with remnant polarization of 5 μC/cm2 and coercive field of 80 kV/cm. Furthermore NBT-BT films showed pyroelectricity with pyroelectric coefficient of 0.6×10−8 C/cm2K. Monolithic-integration of Si electronics and lead-free ferroelectric NBT thin films has been archived using SiN passivation layer. It was previously believed that LSI processes could not incorporate any sodium-containing material which would cause characteristic degradation, such as threshold voltage shift. In this work, no threshold voltage shift in MOS characteristics was observed using this SiN layer. The SiN layer not only blocked diffusion from NBT chemistry, but also from crystallized NBT films during NBT formation process.

Pulsed laser deposition of lead-free (Na0.5Bi0.5)1−xBaxTiO3 ferroelectric thin films with enhanced dielectric properties

Ferroelectric lead-free (Na0.5Bi0.5)1−xBaxTiO3 thin films obtained by pulsed laser deposition have been structurally and electrically investigated for compositions, x=0 and x=0.06, in and out of the morphotropic phase boundary (MPB). Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT), pure or in solid solution with other materials (like BaTiO3), is considered to be the best candidate material for lead-free ferroelectric and piezoelectric applications such as actuators and nonvolatile memory devices. Bulk solid solutions with BaTiO3 (BT), (1−x)NBT–xBT (NBT–x%BT) have been investigated widely, also due to a morphotropic phase boundary (MPB) with enhanced dielectric and ferroelectric properties between a rhombohedral and a tetragonal ferroelectric phase, at x=0.06. Nonetheless, to transpose bulk properties to NBT–BT thin films is a major achievement. XRD technique has been used for structural characterizations of NBT–BT films. Dielectric spectroscopy measurements were performed at room temperature in the frequency range 100Hz–1MHz. The best films show pure perovskite phase and good crystalline structure, as a function of specific deposition conditions. Unusual characteristics, especially dielectric constant values higher than those for bulk, have been found for films with specific crystallographic orientations.

Nanostructured lead-free ferroelectric Na0.5Bi0.5TiO3–BaTiO3 whiskers: synthesis mechanism and structure

Nanostructured lead-free ferroelectric Na(0.5)Bi(0.5)TiO(3)-BaTiO(3) (NBTBT) whiskers with a high aspect ratio were synthesized topochemically using Na(2)Ti(6)O(13) (NTO) as a host structure for the first time. High energy X-ray diffraction coupled with an atomic pair distribution function (PDF) and Raman scattering analyses were used to confirm the average structure of the lead-free NBTBT whiskers, which was found to be rhombohedral, i.e. a ferroelectric enabling type. High resolution transmission electron microscopic (HRTEM) analysis revealed local monoclinic-type structural distortions, indicating a modulated structure at the nanoscale in the morphotropic phase boundary (MPB) composition of the lead-free NBTBT whiskers. The structural rearrangement during the synthesis of the lead-free NBTBT whiskers was found to occur via translation of the edge shared octahedra of NTO into a corner sharing coordination. High temperature morphological changes that depict the disintegration of the isolated whiskers into their individual grains due to the higher grain boundary energy have been found to occur in close analogy with Rayleigh-type instability.

Crystal growth and electric properties of lead‐free NBT‐BT at compositions near the morphotropic phase boundary

AbstractLead‐free piezoelectric materials such as Na0.5Bi0.5TiO3‐BaTiO3 (NBBT) solid solution have been paid increasing attention from an environmental perspective. In this paper, a series of NBBT single crystals with compositions near the morphotropic phase boundary (MPB) were grown by the top‐seeded solution growth (TSSG) method. The compositional dependence of crystalline structure was investigated by high‐resolution X‐ray diffraction (XRD). In addition, the dielectric, piezoelectric and ferroelectric (FE) properties of NBBT crystals were also investigated. Excellent piezoelectric properties of NBBT crystals, especially large thickness coupling factors kt, were obtained near the MPB, which are more than or close to the performances of PZT ceramics. Moreover, the performances of NBBT crystals can be further enhanced by doping. Finally, the domain configurations of NBBT96/4 crystal were studied preliminarily. magnified image

Lead-free ferroelectric thin films obtained by pulsed laser deposition

Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) thin films grown by pulsed laser deposition have been investigated by X-ray diffraction, scanning electron microscopy, and dielectric spectroscopy in order to clarify the role of substrate temperature on crystalline structure, grain morphology, and dielectric properties. We have shown that the structural and dielectric properties of NBT-BT thin films with composition at morphotropic phase boundary (6% BT) critically depend on the substrate temperature: small variations of this parameter induce structural changes, shifting the morphotropic phase boundary toward tetragonal or rhombohedral side. Higher deposition temperature (1000 K) favor the formation of rhombohedral phase, films deposited at 923 K and 973 K have tetragonal symmetry at room temperature. Grains morphology depends also on the deposition temperature. Atomic force micrographs show grains with square or rectangular shape in a compact structure for films grown at lower temperatures, while grains with triangular shape in a porous structure are observed for films grown at 1000 K. Dielectric spectroscopy measurements evidenced the phase transition between ferroelectric and antiferroelectric phase at 370 K. Films grown at 1000 K shown low electrical resistivity due to their porous structure. High dielectric constant values (about 800 at room temperature and 2700 at 570 K) have been obtained for films grown at temperatures up to 973 K.

LEAD-FREE (Na0.5Bi0.5)1-xBaxTiO3 SINGLE CRYSTALS (0 ≤ x ≤ 0.05) AND THEIR DIELECTRIC AND PYROELECTRIC PROPERTIES

ABSTRACT Lead-free single crystals of Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) system were prepared by Czochralski method and their dielectric and pyroelectric properties have been studied. The results were compared with these for pure NBT. The obtained samples show pure perovskite structure with rhombohedral symmetry at room temperature. Low frequency (100 Hz–100 kHz) investigations reveal the diffuse phase transitions. The obtained results are discussed in terms of local electric and strain fields caused by different ionic radius of (Na, Bi) and Ba ions. The NBT-BT system is expected to be a new promising candidate for lead-free electronic crystals.

Grain growth kinetics of textured 0.92Na0.5Bi0.5TiO3—0.08BaTiO3 ceramics by tape casting with Bi2.5Na3.5Nb5O18 templates

Plate-like Bi2.5Na3.5Nb5O18 particles were used as templates to fabricate grain-oriented Na0.5Bi0.5TiO3—BaTiO3 (NBTBT) ceramics by reactive-templated grain growth. The effects of sintering conditions on the grain orientation and microstructure of the textured NBTBT ceramics were investigated, and the kinetic mechanism of grain growth is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.6. NBTBT specimens are composed of strip-like grains and equiaxed shaped grains. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The degree of grain orientation increases initially, then decreases with increasing sintering temperature and soaking time. The maximum texture fraction is 0.69 when sintered at 1185 °C for 6 h. The kinetic exponent n and activation energy Q of the two types of grain in textured NBTBT ceramics were calculated. The results show that the grain growth mechanism of oriented grains is controlled not only by grain lattice diffusion, but also by grain boundary diffusion. The grain growth mechanism of matrix grains is mainly controlled by the grain boundary curvature.

Fabrication and dielectric properties of textured Na0.5Bi0.5TiO3-BaTiO3 ceramics by RTGG method

Grain-oriented Na0.5Bi0.5TiO3-BaTiO3 (NBTBT) ceramics were fabricated by reactive-templated grain growth using plate-like Bi2.5Na3.5Nb5O18 (BNN) as templates. The specimens are composed of NBTBT perovskite phase and BNN lay-structured phase. Textured ceramics have a brick-wall microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The texture fraction increases initially, and then decreases with increasing sintering temperature. The optimal sintering temperature is 1,185 °C where the texture fraction has a maximum value of 0.58 and d33 is 98 pC/N. The textured NBTBT ceramics show evidence of relaxor ferroelectrics with diffuse phase transition and frequency dispersion because of composite biphasic structure.

The Effect of Axial Pressure on Domain State and Dielectric Properties of Na0.5Bi0.5TiO3 and Related Materials

The domain phenomena measurements of Na0.5Bi0.5TiO3 (NBT) and Na0.5Bi0.5TiO3-BaTiO3 (NBT-BT) under axial pressure have been performed. It was found that the measured materials are sensitive to external stress. This is connected with: (i) the shift of the phase transition temperature, (ii) decrease of permittivity and reduction of thermal hysteresis, and (iii) change in domain structure. It was shown that it is possible to obtain a stable ferroelastic monodomain state under axial pressure < 100 bar. Taking into account a larger value of coercive field and to big external stresses—the investigated materials can be used in severe conditions.