Domain Structure of Potassium‐Sodium Niobate Ceramics Before and After Poling

Domain structure and domain wall motion play important roles on the piezoelectric properties of ferroelectric ceramics. In this work, the domain structure of hot‐pressed (K_0.50Na_0.50)NbO₃ (KNN) ceramics before and after poling were studied by observing the domain patterns with an acid‐etching technique, and the extrinsic contribution to the piezoelectric properties were evaluated. It was found that the domain structure of the unpoled KNN ceramic was relatively complicated with many watermark, herringbone and zigzag patterns, while only a single set or few sets of parallel domain stripes were observed in the poled KNN ceramic, due to the domain reorientation and domain wall motion during the poling process. The average domain width changes from 200 (±10) nm before poling to 250 (±10) nm after poling. Domain configurations of “Herringbone‐Zigzag‐Watermark” and “Herringbone‐ Herringbone‐Zigzag” types observed in the unpoled KNN ceramic were then further analyzed. The extrinsic contribution to the piezoelectric properties from the domain reorientation and irreversible domain wall motion in the hot‐pressed KNN ceramic was found to be 71%, slightly higher than that of conventional sintered KNN ceramics ~68%.     

The temperature-dependent piezoelectric and electromechanical properties of cobalt-modified sodium bismuth titanate

Lightly cobalt-modified, Aurivillius-type, sodium bismuth titanate (Na_0.5Bi_4.5Ti₄O₁₅, NBT) ceramics were synthesized by substituting a small amount of cobalt ions onto the Ti⁴⁺ sites using conventional solid-state reaction. X-ray photoelectron spectroscopy (XPS) analysis coupled with bond valence sum calculations show that the dopant cobalt ions substitute for Ti⁴⁺ ions in the form of Co³⁺. The resultant cobalt-modified NBT ceramics (NBT-Co) exhibit better piezoelectric and electromechanical properties by comparison with pure NBT. With only 0.3 wt% Co³⁺ substitution, the piezoelectric properties of the NBT-Co ceramics are optimal, exhibiting a high piezoelectric coefficient (d₃₃~33 pC/N), a low dielectric loss tan δ (~0.1% at 1 kHz), a high thickness planar coupling coefficient (k_t~34%) as well as a high Curie temperature (T_c~663 °C). Such NBT-Co ceramics exhibit nearly temperature-independent piezoelectric and electromechanical properties up to 400 °C, suggesting that these cobalt-modified NBT ceramics are promising materials for high temperature piezoelectric applications.     

Piezoelectric properties and microstructures of ZnO-doped Bi0.5Na0.5TiO3 ceramics

This article studies the microstructure and piezoelectric properties of a ceramic lead-free NBT under different amount of ZnO doping. X-ray diffraction shows that Zn²⁺ diffuses into the lattice of (Bi_0.5Na_0.5)TiO₃ to form a solid solution with a pure perovskite structure. By modifying the zinc oxide content, the sintering behavior of (Bi_0.5Na_0.5)TiO₃ ceramics was significantly improved and the grain size was increased. The piezoelectric coefficient d₃₃ for the 1.0 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1050 °C was found to be 95 pC/N, and the electromechanical coupling factor k_p = 0.13. However, the piezoelectric coefficient d₃₃ for the 0.5 wt.% ZnO-doped (Bi_0.5Na_0.5)TiO₃ ceramics sintered at 1140 °C was found to be 110 pC/N, and the electromechanical coupling factor k_p = 0.17.     

Poling effect on optical and dielectric properties of Pr3+-doped Na0.5Bi0.5TiO3 ferroelectric single crystal

In recent years, with increased environmental awareness, more and more research has been carried out for lead-free ferroelectric materials. Among such materials, Na_0.5Bi_0.5TiO₃ (NBT) single crystal has become a material of interest due to its high Curie temperature. In this study, Pr-doped NBT crystals were prepared and poled along different crystal orientations. The effects of different crystal orientations and poling on the optical properties of Pr-doped NBT crystals were investigated. The optical transmission of the crystal decreases after poling along the [001] orientation and increases after poling along the [111] orientation. The emission and excitation spectra show that the poled process enhances the crystal luminescence. The dielectric constant dependence on temperature for different crystal orientations was measured. The domain structure was measured to explain the changes in the optical properties. The optical properties of a Pr-doped NBT single crystal were modulated by poling along different crystal orientations, which is expected to improve the performance of optoelectronic devices based on such materials.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

Enhanced piezoelectric,electrocaloric and energy storage properties at high temperature in lead-free Bi0.5(Na1-xKx)0.5TiO3 ceramics

The piezoelectric, electrocaloric and energy storage properties were systemically investigated in lead-free Bi_0.5(Na_1-xK_x)_0.5TiO₃ ceramics from room temperature to high temperature region. These ceramics can be poled completely to obtain large piezoelectric coefficient (104–153 pC/N) at low electric field of ~30?kV/cm. The piezoelectric property shows good thermal stability due to high depolarization temperature (T_d). For BNKT₂₀, a large low electric field-induced strain of 0.36% is obtained at 120?°C under 50?kV/cm, the corresponding normalized strain coefficient is up to 720?pm/V, which is larger than other BNT-based ceramics at high temperature region. The electrocaloric properties of these ceramics are studied via indirect and direct methods. Large EC value (~1.08?K) in BNKT₂₀ ceramic is obtained at 50?kV/cm using indirect calculation. Above 100?°C, the dielectric energy storage density and efficiency of BNKT₂₀ is still up to ~0.85?J/cm³ and 0.75, respectively. The BNKT_x ceramics may become promising candidates in the fields of actuators, electrocaloric cooling and energy storage at high temperature region.     

Low‐temperature sintering and enhanced piezoelectric properties of random and textured PIN–PMN–PT ceramics with Li2CO3

Low‐temperature sintered random and textured 36PIN–30PMN–34PT piezoelectric ceramics were successfully synthesized at a temperature as low as 950°C using Li₂CO₃ as sintering aids. The effects of Li₂CO₃ addition on microstructure, dielectric, ferroelectric, and piezoelectric properties in 36PIN–30PMN–34PT ternary system were systematically investigated. The results showed that the grain size of the specimens increased with the addition of sintering aids. The optimum properties for the random samples were obtained at 0.5 wt% Li₂CO₃ addition, with piezoelectric constant d₃₃ of 450 pC/N, planar electromechanical coupling coefficient k_p of 49%, peak permittivity ε_max of 25 612, remanent polarization P_r of 36.3 μC/cm². Moreover, the low‐temperature‐sintered textured samples at 0.5 wt% Li₂CO₃ addition exhibited a higher piezoelectric constant d₃₃ of 560 pC/N. These results indicated that the low‐temperature‐sintered 36PIN–30PMN–34PT piezoelectric ceramics were very promising candidates for the multilayer piezoelectric applications.     

Phase-composition dependent domain responses in (K0.5Na0.5)NbO3-based piezoceramics

Lead-free (K_0.5Na_0.5)NbO₃-based (KNN) piezoceramics featuring a polymorphic phase boundary (PPB) between the orthorhombic and tetragonal phases at room temperature are reported to possess high piezoelectric properties but with inferior cycling stability, while the ceramics with a single tetragonal phase show improved cycling stability but with lower piezoelectric coefficients. In this work, electric biasing in-situ transmission electron microscopy (TEM) study is conducted on two KNN-based compositions, which are respectively at and off PPB. Our observations reveal the distinctive domain responses in these two ceramics under cyclic fields. The higher domain wall density in the poled KNN at PPB contributes to the high piezoelectric properties. Upon cycling, however, a new microstructure feature, “domain intersection”, is directly observed in this PPB composition. In comparison, the off-PPB KNN ceramic develops large domains during poling, which experience much less extent of disruption during cycling. Our comparative study provides the basis for understanding the relation between phase composition and piezoelectric performance.     

Structural and electrical properties of 0.57PSN-0.43PT ceramics prepared by mechanochemical synthesis and sintered at low temperature

In this investigation we show that the dielectric, ferroelectric and piezoelectric properties of stoichiometric 0.57Pb(Sc_1/2Nb_1/2)O₃-0.43PbTiO₃ (0.57PSN-0.43PT) ceramics prepared by mechanochemical synthesis are comparable or even better than the properties of 0.57PSN-0.43PT ceramics with Nb doping, which was proposed to enhance the electrical properties. Here, the stoichiometric ceramic was sintered to 97% of theoretical density at a temperature of 1000 °C, which is 200-300 °C lower than previously reported. The dielectric constant ?, remnant polarization P_r, piezoelectric coefficient d₃₃, coupling coefficients k_p and k_t and mechanical quality factor Q_m of the ceramics prepared by mechanochemical synthesis were 2200, 43 μC/cm², 570 pC/N, 0.71, 0.56 and 38, respectively.The effects of the poling field on the structural and electrical properties of the 0.57PSN-0.43PT ceramics were investigated. The results show that the ratio of the monoclinic to the tetragonal phases is influenced by the application of the poling electric field. The non-poled ceramics contain 71% of the monoclinic phase and 29% of the tetragonal phase. The highest d₃₃, k_p and k_t were measured for ceramics poled at an electric field of 3 kV/mm. For these poled ceramics a phase determination of 86% monoclinic phase and 14% tetragonal phase was obtained from Rietveld refinements.     

Ca2+ doping effects on the structural and electrical properties of Na0.5Bi4.5Ti4O15 piezoceramics

Bismuth layer structured Na_0.5Bi_4.5Ti₄O₁₅ (NBT) ferroelectric is one of the most promising materials for potential applications at high temperature. However, it is challenged to achieve a balance between high Curie temperature piezoelectric coefficient and excellent thermal stability for NBT piezoceramics. Here, through chemical modification at the A site of NBT with Ca²⁺, novel (Na_0.5Bi_0.5)_1-xCa_xBi₄Ti₄O₁₅ piezoceramics with excellent properties fabricated by solid state reaction were studied. After doping of Ca²⁺, the Curie temperature T_C increased from 648 °C to 662 °C while the piezoelectric coefficient d₃₃ increased from 14 pC/N to 22 pC/N which can be attributed to the intrinsic contribution of TiO₆ octahedral lattice distortion (tilting and rotation) and the extrinsic contribution of the increased density of domain walls. The composition of (Na_0.5Bi_0.5)_0.95Ca_0.05Bi₄Ti₄O₁₅ ceramics with x = 0.05 has the optimal performance with high T_C of 655 °C, large d₃₃ of 22 pC/N, high electrical resistivity ρ close to 10⁷ Ω cm at 500 °C and especially excellent thermal stability of d₃₃ only about 5% reduction after being annealed at 625 °C. The work effectively reveals the great potential of CNBT-5 ceramics for high-temperature piezoelectric applications.     

Densification and properties of oxygen sintered CuO-doped PIN-PMN-PT ceramics

Relationships between sintering temperature and annealing atmosphere on microstructure and dielectric, ferroelectric, and piezoelectric properties of reactively sintered CuO-doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) ceramics were investigated. Uniform 2−3 μm grain size, dense CuO-doped PIN-PMN-PT ceramics are obtained when oxygen sintered versus a bimodal grain size microstructure when sintered in air. Oxygen sintered ceramics have excellent properties including d₃₃ = 300–315 pC/N, E_C = 7.7–8 kV/cm, and tan δ < 1.5%. The MPB region was mapped for ternary compositions doped with 0.5 mol% CuO and sintered in O₂. MPB 25PIN-40PMN-35PT demonstrated the maximum piezoelectric properties with d₃₃ = 565 +/− 23 pC/N and k_p = 0.64 +/− 0.01. Sintering from 1050 °C to 1200 °C increased the coercive field from 8.5 to 11.5 kV/cm and reduced dielectric losses from tan δ = 1.8% to 0.8% by facilitating diffusion of CuO into the lattice and creating domain wall pinning defect dipoles as evidenced by an increase in the internal field bias of P-E loops.     

Correlation between grain size and electrical properties of high-temperature lead-free 0.70BiFeO3-0.30BaTiO3 ceramics

0.70BiFeO₃-0.30BaTiO₃ (0.70BF-0.30BT) ceramics have been widely concerned because of their potential applications for high-temperature piezoelectric devices. In this work, a series of dense 0.70BF-0.30BT ceramics with average grain size variation from 0.55 to 6.0 μm were prepared. XRD results indicate that 0.70BF-0.30BT ceramics show the coexistence of rhombohedral and pseudo-cubic phases and the volume fraction of the rhombohedral phase increase with the grain size. The dielectric, ferroelectric and piezoelectric properties increase with the grain size initially from 0.55 to 5.0 μm and then decrease slightly. Values of d₃₃, P_r, and ε_r, of 0.70BF-0.30BT ceramics with the grain size of 5.0 μm are 185 pC/N, 21.2 μC/cm², and 638, respectively, about five times higher than those ceramics with fine-grain of 0.55 μm. Of particular importance is that 0.70BF-0.30BT ceramics with large grain sizes possess better piezoelectric thermal stability due to the much stabler poled domain state with the rising temperature. The detailed structural studies indicate that the enhanced electric properties are owing to the significantly improved domain motion and the increased lattice distortion. This clarifying the relationship between electrical properties and grain size offers a novel way of improving the performances of piezoceramics.     

Investigation of the mechanism of domain switching in different Na0.5Bi0.5TiO3 microstructures

Several researches have studied the physical properties of hydrothermally-synthesized low dimensional piezoelectric nanostructures. However, the obtained piezoelectric coefficient is not high and the relationship between physical properties and microstructures is still neglected. Here we report the piezoelectric and ferroelectric properties of different lead-free sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) microstructures synthesised with hydrothermal routes and give visualization of domain structures using piezoresponse force microscopy. The NBT nanowire exhibits better local piezoelectric response compared with NBT spherical aggregates and microcubes and other one-dimensional materials prepared by hydrothermal method and the large piezoelectric coefficient of nanowire was explained by observed regular stripe domain structures. Moreover, it is found that there are different domain configurations at the top and side of the nanowire under the external electrical fields, which don't change the regular stripe domain structure but lead to the movement of domain boundaries. By finite element modeling, it attributes to the different electric potential distributions from tip within the nanowire.     

Structural and Raman spectroscopic studies of poled lead-free piezoelectric sodium bismuth titanate ceramics

Lead-free piezoelectric sodium bismuth titanate (NBT) ceramics are synthesized by a solid state reaction method. Poled NBT ceramics possess rhombohedral R3c structure. Vibrational phonon frequencies associated to Bi–O, Na–O, TiO₆ octahedra and oxygen vibrations are observed. Experimentally unreported lowest frequency region peak centered at 111 cm⁻¹ is assigned to E(TO1) mode related to Bi–O vibrations. Appearance of lowest frequency mode may be due to well-polished surfaces with reduced Rayleigh scattering background and well-defined ordering of Bi and Na ions in the lattice.     

Microstructural and mechanical behavior of Na0.4K0.1Bi0.5TiO3 ferroelectric ceramics

Na_0.4K_0.1Bi_0.5TiO₃ (NKBT) solid solution was synthesized by the conventional solid-state route. X-ray diffraction pattern of poled NKBT ceramic was refined by the Rietveld technique to determine the structural phases (Rhombohedral and Tetragonal) present in the solid solution. High density (98% of theoretical density) ceramics exhibited an average grain size of 1.2 μm from the SEM analysis. EDS spectra and elemental mapping were recorded to confirm the formation of exact stoichiometric composition in the prepared ceramic. The hardness of the NKBT ceramic was measured by nano-indentation and Vickers microhardness techniques. Young's modulus was measured by nano-indentation, ultrasonic echo pulse tester, compression tests and compared. The effect of mechanical stress on piezoelectric properties of pre-poled NKBT ceramic was studied at room temperature. Fracture toughness (1.45 MPa m^1/2) and compressive strength (250 MPa) of the ceramic were determined to ensure its candidature for actuator application from a mechanical perspective. The phenomena of mechanical depolarization were explained through the domain rotation concept.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

Enhancement of piezoelectric catalysis of Na0.5Bi0.5TiO3 with electric poling for dye decomposition

In this work, the performance of solid-phase sintered Na_0.5Bi_0.5TiO₃ ceramic powder catalyst is investigated for Rhodamine B (RhB) dye decomposition under different electric poling fields. As a result of the vibration excitation, the piezoelectric catalytic decomposition ratio of the poled Na_0.5Bi_0.5TiO₃ for RhB dye wastewater increases from 10.84% to 51.32% as the electric poling field increases from 0 to 0.5 kV/mm. The improved piezoelectric catalytic activity could be attributed to a rise in the piezoelectric performance of Na_0.5Bi_0.5TiO₃ after electric poling. The addition of the different types of radical scavengers in piezoelectric catalysis further proves that a large amount of superoxide-free radicals and a small number of hydroxyl radicals participate in the catalytic reaction. Our results demonstrate that electric poling, as a simple and effective method of improving piezoelectric catalysis, has potential applications in dye-containing wastewater treatment.     

Enhanced electrical properties of Cr2O3 addition NBT-based high-temperature piezoelectric ceramics

Owing to industrial and technological developments, there has been an increasing demand for piezoelectric ceramics that can function at temperatures of 500°C or higher. Na_0.5Bi_4.5Ti₄O₁₅ (NBT) with its high Curie temperature (T_C) of 650°C is a typical bismuth layer–structured ferroelectric. However, its relatively low piezoelectric coefficient (d₃₃ ∼ 16 pC/N) hinders its potential application at high temperatures. In this study, compositions of Ca_0.05(Na_0.5Bi_0.5)_0.95Bi₄Ti₄O₁₅ with different additions of Cr₂O₃ (CNBT–Cr100x) were designed based on previous studies on Ca²⁺-doped NBT piezoceramics, and the effects of the addition on the structural and electrical properties were investigated. The d₃₃ value of CNBT–Cr20 was as high as 29 pC/N, almost twice higher than that of pure NBT ceramics. This increase was investigated in depth using X-ray diffraction refinement and piezoelectric force microscopy in terms of intrinsic and extrinsic contributions. The P_s values of CNBT and CNBT–Cr20 were almost equal. The density of the domain walls of CNBT–Cr20 was significantly higher than that of CNBT, indicating that the increase of d₃₃ of CNBT–Cr20 is mainly due to the increase in the extrinsic contribution. The CNBT–Cr20 ceramic exhibited excellent properties with a high T_C of 655°C, a high d₃₃ of 29 pC/N, and a resistivity high than 10⁶ Ω cm at 500°C, demonstrating its potential for applications at high temperatures such as 500°C.     

Aging effect in Er3+-doped 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 ceramics

Usually, aging in poled ferroelectrics leads to degradation of certain physical properties. In this study, we found a remarkable aging effect in tetragonal Er³⁺-doped 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-BCT) ceramics after poling. It is observed that the domains can spontaneously rotate to keep their spontaneous polarization direction similar to that of the poling electric field during aging for the poled ceramics. Furthermore, compared with freshly poled ceramics, the thermally stimulated current (TSC) peak of the aged ones shifts toward a higher temperature (10°C). And the temperature of the TSC peak in the aged ceramics is exactly equal to their Curie temperature. Such features indicate that aging for the poled ceramics could stabilize the alignment of ordered ferroelectric domains. Additionally, a downward TSC peak above Curie temperature is obtained in both poled and aged ceramics, demonstrating that poling and aging can lead to ordered alignment of defect dipoles. The aging mechanism of poled Er³⁺-doped BZT-BCT ceramics has proposed and discussed in this article.     

Analysis of the rhombohedral–tetragonal symmetries coexistence in lead-free 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics from nanopowders

Ceramics with perovskite-type structure and 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (BNBT) composition have been studied by conventional powder X-ray diffraction in Bragg–Brentano geometry. Ceramics were obtained from sol–gel autocombustion nanopowders and processed either by hot pressing and subsequent recrystallisation or pressureless sintering in two steps. These methods provided single-phase, sub-micron grain size (<700?nm), dense ceramics with good piezoelectric performance (96–94% of theoretical density and d₃₃?=?143–124?pC?N^–1, respectively). For the considered ceramics, the splitting of the peaks of the cubic perovskite-type structure with 111 and 200 Miller indices has been repeatedly used as a symmetry identification criterion. In this work a simple, yet powerful, procedure to validate the consistency of the mentioned splitting interpretation is presented. Based on peaks fitting and well-known crystallographic expressions, the rhombohedral and tetragonal symmetries' coexistence is verified. The suggested procedure can be applied to the study of peak splitting in ceramics at Morphotropic Phase Boundaries in a general way. In a given series of BNBT ceramics, inconsistencies for interplanar distances, intensities' ratios and the evolution of these from not-poled to poled samples have been found. In poled ceramics, special care has been taken when carrying out this analysis, due to the anisotropic strains arising from ferroelectric (FE) domain orientation. Poling gives rise to a displacement of the peaks angular positions and modification of the intensity ratios. However, the interplanar distance changes associated with the angular deviations here observed are one order of magnitude higher than those expected from anisotropic strains. These results set up a doubt on the sufficiency of the [rhombohedral?+?tetragonal] model to characterise the considered ceramics. A model of a mesoscopic FE phase with rhombohedral symmetry, a mesoscopic and globally weakly polar phase, with cubic symmetry, and a nanosised phase, also cubic, is presented as a plausible alternative.