Lead-free Na0 Research Articles

Tailoring the structural and optical properties of lead-free Na0.5(Bi1-xSmx)0.5TiO3 multifunctional ceramics by Sm-doping

In this study, the effects of Sm3+-content on the structural and optical properties of the Na0.5(Bi1-xSmx)0.5TiO3 multifunctional lead-free system were investigated using x-ray diffraction (XRD), ultraviolet-visible (UV-Vis), Raman spectroscopy and photoluminescence (PL) spectroscopy. The Na0.5(Bi1-xSmx)0.5TiO3 (x = 0.000, 0.005, 0.010, 0.015, and 0.020) ceramics were synthesized by the solid-state reaction method with subsequent annealing. The Rietveld method was used to refine the XRD data. The results of these refinements indicated that pure NBT (Na0.5Bi0.5TiO3) and the Sm3+-doped samples showed the coexistence of the rhombohedral (R3c) and tetragonal (P4bm) phases in different fractions, which depend on the Sm3+-content. The UV-Vis data revealed that the optical band gap of the Na0.5(Bi1-xSmx)0.5TiO3 samples monotonically decreases with increasing Sm3+-content, providing evidence of replacement of Bi3+- by Sm3+-ions. Furthermore, Raman spectroscopy confirmed that substitution of Bi3+- by Sm3+-ions leads to a shortening of the Na/Bi-O bond length. It was demonstrated that this behavior is responsible for the increase in the local symmetry of the TiO6 octahedron and the consequent decrease in the ratio between both rhombohedral and tetragonal phases, as observed by the XRD data. The findings from the PL data revealed typical emissions corresponding to transitions between the 4G5/2 → 6Hj (j = 5/2, 7/2, 9/2, 11/2, 13/2) states of the Sm3+-ion. Additionally, the presence of non-radiative recombination processes was found to depend on both laser pumping power and Sm3+-content.

Solid state synthesis of lead-free Sm-doped Na0.5Bi4.5Ti4O15 ceramics for enhanced dielectric and electrical properties

Here we report the synthesis of Sm-doped Na0.5Bi4.5Ti4O15 (Na0.5Sm0.5Bi4Ti4O15) lead-free ceramics via a conventional solid-state technique. Investigations of Na0.5Bi4.5Ti4O15 (NBT) and Na0.5Sm0.5Bi4.5Ti4O15 (NSBT) ceramics were demonstrated in detail to understand the composition-based structure-property of Aurivillius compounds and related functional material. Dielectric properties for frequency and temperature in a wide range were analyzed. The conduction activation energy values of NSBT ceramics are obtained to be 1.40 eV, whereas, the NBT ceramics get the value to be 1.31 eV. At higher temperatures, the conduction activation energy value of NSBT ceramics is 1.32 eV for both frequencies of 100 Hz and 1 kHz, whereas, for NBT compounds, the calculated value is 1.27 eV for both frequencies. The simulation performed on the impedance data for capacitive and resistance elements shows well-fitting curves which indicates a single relaxation behavior in the material. Similarly, the AC-conductivity data were analyzed which gives different conduction processes and relaxation activation energies in the NSBT ceramics.

Optimizing a machine learning design of dielectric properties in lead-free piezoelectric ceramics

Designing lead-free piezoelectric ceramics with tailored electrical properties remains a critical challenge for various applications. In this paper we present a novel methodology integrating Machine Learning (ML) and optimization procedures to fine-tune electrical properties in lead-free (1-x) Na0.5 Bi0.5 TiO3 - x CaTiO3 piezoelectric ceramics. A comprehensive dataset of dielectric measurements serves as the foundation for training ML models that accurately predict the permittivity (ε′) and dielectric loss (tan δ) as functions of Ca2+ concentration (x % Ca), temperature and frequency. Two ML techniques are evaluated: random forest regression, and Multi-Layer Perceptron neural network Regression (MLPR). The MLPR model exhibited a superior regression performance, achieving a correlation coefficient of 0.931 and a root mean squared error of 0.029. The MLPR was then optimized by the Non-dominated Sorting Genetic Algorithm II (NSGA-II) to maximizes ε′ while minimizes tan δ. Within the NSGA-II framework, the optimal values were found at the Pareto curve knee, corresponding to a frequency, temperature, and x % Ca of 609.739 kHz, 398.15 K, and 6.10, respectively, resulting in ε′ equal to 857.87 and tan δ equal to 0.0120. This approach demonstrates the effectiveness of combining ML and optimization for designing the electrical properties of piezoelectric ceramics, paving the way for more efficient and targeted material development.

Investigations of energy storage and thermal stability properties in eco-friendly B-site substituted Na0.5Bi0.5TiO3

In this work, (Mg1/3Nb2/3)4+ cationic substitution in lead-free polycrystalline Na0.5Bi0.5TiO3 (NBT) has been adopted to synthesize by the conventional solid-state reaction method. The cationic substitution was introduced into NBT through compositional modification, which significantly restricted grain growth and promoted a ferroelectric to relaxor ferroelectric phase transition. The substitution of (Mg1/3Nb2/3)4+ influenced the crystal structure, ferroelectric, dielectric properties, and micromorphology, which had been investigated systematically. The enhancement of relaxor nature is found with this substitution and further verified by dielectric curve, polarization (P) - electric field (E) hysteresis loop, and current (I)- electric field (E) curve. The recoverable energy density was determined to be 1.09 J/cm3 and 1.24 J/cm3 at 101 kV/cm and 114 kV/cm in the 10 mol% and 20 mol% of (Mg1/3Nb2/3)4+ substituted systems, respectively. Furthermore, over the 35 ˚C to 490 ˚C temperature range, the 30 mol% (Mg1/3Nb2/3)4+ substituted composition showed superior thermal stability in the permittivity. Therefore, this study concluded that these cationic substituted NBT compositions are appropriate materials for applications involving higher-temperature and energy storage in electronic devices.

Effect of Mn (as an acceptor) on the structural and electrical properties of non-stoichiometry Sodium Bismuth Titanate/NBT ceramics

The lead free Na0.5Bi0.5TiO3 (NBT) ceramic is proven as a potential piezoelectric candidate; however, non-stoichiometry NBT serves as an outstanding oxide-ion conductor proposed by (M. Li et al., Nature Materials 13 (2014) 31–35). According to this point here we have systematically report the structural and electrical properties of non-stoichiometry Na0.5Bi0.49TiO3/NB0.49T and B-site Mn3+ acceptor-modified NB0.49T system. Therefore, we are trying to prepare the nominal formula of Na0.5Bi0.49Ti1-xMnxO3; NB0.49T ceramic with 0.0≤x ≤ 0.05 by solid state reaction method. The bulk oxide ion conductivity gradually increases with respect to the doping concentration and the maximum value was obtained for x = 0.05 composition. This conductivity maximum is in good consistent with R. A. De Souza's oxygen vacancy diffusivity limit concept for perovskite systems (Advanced Functional Materials, 25 (2015)). On doping of 5mol% of Mn in the Ti-site/B-site NB0.49T system, it was observed that the oxygen-ion conduction inside the grain boundary increased considerably. Grain conductivity of NaBi0.49TiO3 at 500 °C is ∼3.0 ± 0.5 mS cm−1, which is approximately 122 % greater than that pure NBT compositions. Additionally, NB0.9T -exhibited a conductivity value of 4.7 mS/cm at 500 °C, which increases to 9.2 mS/cm on 5 mol% Mn-modification. The observed increase in conductivity was ascribed due to the electrostatic interactions between Mn–Ti or Mn–O bond. Therefore, our current findings on NB0.49T illustrates that present approach should be applicable to modify the system for SOFC application.

Na0.5Bi0.5TiO3 perovskite anode for lithium-ion batteries

This work reports the use of lead-free Na0.5Bi0.5TiO3 perovskite as a lithium-ion battery anode and explores its charge storage mechanism. It opens the door to explore numerous Bi-based oxides capable of reversibly hosting lithium.

High-entropy (Na0.2Bi0.2Ba0.2Sr0.2Zn0.2)TiO3 ceramics with enhanced energy storage density and reliable stability

High-entropy ceramics (HECs) have gained increasingly recent interest due to their fantastic physiochemical properties and rich compositions. The investigation of energy storage performance based on the temperature and frequency stability of HECs is still at an early stage, with limited reported studies. Here, we report lead-free (Na0.2Bi0.2Ba0.2Sr0.2Zn0.2)TiO3 (NBBSZT) HECs by a solid-state approach with a pressureless sintering process. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies demonstrate that NBBSZT HECs show a pure perovskite structure, uniform morphology and element distribution. Ferroelectric measurements indicate that the NBBSZT HECs exhibit an improved energy storage density of 1.03 J/cm3 and an efficiency of 77%, which is approximately 5 times and 17 times enhancement compared to Bi0.5Na0.5TiO3 (BNT) ceramics, respectively. The NBBSZT HECs also demonstrate excellent thermal stability and satisfactory frequency stability. More interestingly, the dielectric property studies show that the NBBSZT HECs exhibit frequency dispersion and diffuse phase transition. Temperature-dependent dielectric measurements indicate that the NBBSZT HECs exhibit better relaxor characteristics than that of BNT ceramics. These studies reveal that the high-entropy strategy provides a potential platform for designing ferroelectric ceramics.

Energy storage properties of KNb0.6Ta0.4O3 modified (Na0.5Bi0.5)0.7Sr0.3TiO3 ceramics

Lead-free (1–x)(Na0.5Bi0.5)0.7Sr0.3TiO3–xKNb0.6Ta0.4O3 ((1−x)NBST–xKNT, x = 0–0.3) ceramics were synthesized via citrate combustion process. The addition of KNT reduces the grain size, decreases the remanent polarization (P r), and slimmer hysteresis loops are realized. (1−x)NBST–xKNT ceramics exhibit typical relaxor characteristics as x increases. A large recoverable energy storage density (W re ∼ 1.74 J/cm3) and high efficiency (η ∼ 87%) are obtained for 0.8NBST–0.2KNT ceramic sample at 150 kV/cm. The energy-storage performances of 0.8NBST–0.2KNT sample show excellent temperature stability (W re varies 9.7% and η varies 2.3% within 25–75 °C).

The influence of Bi nonstoichiometry on the depolarization temperature in lead-free piezoelectric Na0.5Bi0.5+xTiO3+1.5x

The influence of Bi-nonstoichiometry of Na0.5Bi0.5+xTiO3+1.5x (NBT) on the depolarization temperature Td was investigated by in-situ direct piezoelectric measurements (d33) and Resonant Piezoelectric Spectroscopy (RPS). For the latter measurements, the converse piezoelectric effect was used to determine the temperature dependence of the piezoelectric effect and elastic modulus. The composition x = 0.01 shows Td = 137 °C, whereas in x = 0 and x = 0.015 Td = 177 °C. The composition with x = 0 has defects, such as oxygen/bismuth vacancies, related to the loss of Bi2O3 that occurs during high temperature sintering, such as oxygen/bismuth vacancies. At x = 0.01, these defects are minimized via the compensation of Bi2O3 loss, whereas x = 0.015 contains excess Bi2O3. This indicates that minimization of defects decreases Td, while higher Td values are attributed to the shift of the transition temperature between the rhombohedral and the intermediate phases.

Structure, dielectric, and ferroelectric properties of (1-x)Bi0.5Na0.5TiO3 – X K0.5Na0.5NbO3 (0 ≤ x ≤ 0.1) solid solution

The correlation of the phase structure, dielectric, and ferroelectric properties of lead-free (1-x)(Na0.5Bi0.5)TiO3–xK0.5Na0.5NbO3 (NBTKNx) (0 = x ≤ 0.1) polycrystalline ceramics, fabricated via a solid state reaction technique, were investigated. The Rietveld refinement allowed identifying the crystallographic transformation from a rhombohedral to a coexisting rhombohedral-tetragonal or tetragonal long range-ordered ferroelectric (FE) phase. The dielectric investigations showed an increase of the dielectric diffuseness (1.53 = γ ≤ 1.73) and a clear shift of the depolarization temperature (Td) to a lower temperature while increasing substitution. More importantly, the lattice disorder also generated a plateau-like dielectric anomaly, leading to a thermally stable ϵr ∼2859 ± 20% (120–500 °C) and ∼3112 ± 10% (120–420 °C) for x = 0.075 and 0.1 samples, respectively. At room temperature (RT), Raman spectroscopy investigations revealed a downshift of the frequencies as a function of the composition with an inhomogeneous broadening of the Raman lines. On heating, Raman spectra showed changes in the region where the dielectric transitions are observed. Moreover, the composition dependence of the current peaks in the I-E loops confirmed the occurrence of a phase transition from a non-ergodic polar phase to an ergodic weakly polar after the applying of an electric field of 60 kV/cm−1.

Achieving high energy storage performances in high-entropy epitaxial Na0.5Bi0.5Ti0.7Hf0.1Zr0.1Sn0.1O3 thin film

Lead-free Na0.5Bi0.5TiO3 (NBT) exhibiting large polarization and a high Curie temperature can be considered as a promising candidate for dielectric capacitors. The large polarization switching hysteresis and low breakdown field, however, restrict the performance optimization. Herein, epitaxial NBT-based high-entropy Na0.5Bi0.5Ti0.7Hf0.1Zr0.1Sn0.1O3 (NBTHZS) films are designed and prepared by solution-based processing. Compared with the NBT film, the polarization switching hysteresis is depressed and the breakdown field is significantly improved for the NBTHZS film due to the high-entropy effects. Therefore, the NBTHZS film achieves a ∼16 times enhancement of energy density (from 5.1 J/cm3 of the NBT film to 81 J/cm3 of the NBTHZS film) and a high efficiency of 74.1% as well as an excellent performance reliability. The results shed light on enhancing dielectric energy storage properties of NBT-based films by forming high-entropy structures.

Copper nanoparticles-modified Na0.5Bi4.5Ti4O15 micron-sheets as a highly efficient and low cost piezo-photocatalyst

The coupling of piezoelectric effect and metal surface modification is an effective way to improve photocatalytic performance. Herein, we prepared a new piezoelectric heterojunction catalyst by depositing Cu nanoparticles on the surface of lead-free Na0.5Bi4.5Ti4O15 (NBT) piezoelectric micron-sheets. Under the simultaneous excitation of light and stirring, NBT@6.4 wt%Cu photocatalyst exhibits excellent piezo-photocatalytic activity, in which the degradation rate of methyl orange (MO) solution can reach 96% within 120 min, and the rate constant kobs value is as high as 24.67 × 10−3 min−1, which is ∼ 13 times larger than that of pure NBT photocatalyst under the excitation of light. The improved piezo-photocatalytic performance is attributed to that the deposition of Cu nanoparticles and the high piezoelectric properties of NBT facilitates the transfer and separation of photogenerated carriers. This work provides a new option for designing high-performance piezo-photocatalysts for pollutant treatment.

Flexible piezoelectric nanogenerator based on the Er3 + doped lead-free (Na0.5Bi0.5)TiO3-BaTiO3 piezoelectric nanofibers with strong upconversion luminescence

Flexible piezoelectric nanogenerators (PENG) have attracted wide attention because of their great potential in solving the power supply problem of wearable electronic devices. In order to expand the applications of PENG, high quality (Na 0.5 Bi 0.5 )TiO 3 -0.06BaTiO 3 (NBT-0.06BT: x Er 3+ ) nanofibers doped with different Er 3+ concentrations were prepared by the gel-sol electrostatic spinning method. The morphology and phase structure of the prepared nanofibers were characterized. Dense nanofibers with good crystallization have been obtained. The local ferroelectric properties of the nanofibers were characterized by PFM. It is found the prepared nanofibers have a strong upconversion (UC) photoluminescence (PL) property. Utilizing the fluorescence intensity ratio (FIR ) technique, the temperature sensing properties of the nanofibers has been investigated, showing a maximum sensitivity of 0.0043 K −1 . Finally, the flexible PENG was prepared by encapsulating NBT-0.06BT:0.01Er 3+ nanofibers, interdigital electrode and PDMS. The maximum voltage output of PENG is 97 V. The present investigation integrates the luminescence characteristics and electrical properties of nanofibers into the flexible PENG, which may greatly expand its application field. • NBT-0.06BT: x Er 3+ nanofibers with high quality have been successfully prepared. • NBT-0.06BT: x Er 3+ nanofibers have excellent temperature sensing properties. • The present PENG has a potential application in optics. • The output voltage of the PENG reaches a high value of 97 V.

Improved recoverable energy storage density, breakdown strength, and relaxor nature in eco-friendly K+1-ion rich NBT ferroelectrics

Eco-friendly lead-free polycrystalline Na0.5−x Kx Bi0.5TiO3 (x = 0.30,0.35,0.40,0.45) series were synthesized by the solid-state reaction method. The site-specific substitution of K-ion has been performed by varying its concentration. The processing parameters were optimized with increasing K+1-ion percentage. The polarization (P), current (I) and strain (S) behavior with an electric field (E) support the enhanced relaxor nature which is further strengthened by the temperature-dependent dielectric studies. The maximum value of the dielectric constant was found to be 4726 in x = 0.30 composition. The value of Tm is increased from 320°C to 350°C by the substitution of the higher percentage of potassium. The shape of the P-E and I-E loops transforms from pinched type to open type slanted loop. The increased breakdown strength as well as the coercive field with the potassium concentration were confirmed through P-E and I-E hysteresis. The dielectric breakdown has reached close to ∼102 kV/cm for x = 0.45. The optimum estimated recoverable energy storage density and efficiency are found to be ∼0.5 J/cm3 and 58%, respectively, for x = 0.30 among studied compounds. Thus, x = 0.30 composition is found to be suitable for a dielectric capacitor as well as energy storage applications.

Atomic-scale insight into the tetragonal platelets correlated with the antiferroelectricity in Na0.5Bi0.5TiO3-based materials

The structural origin of antiferroelectric (AFE)-like double P–E loops near the depolarization temperature in lead-free Na0.5Bi0.5TiO3 (NBT)-based materials remains elusive despite decades of study. Here, temperature-dependent ferroelectric properties, Raman spectra, and advanced electron microscopy reveal that the AFE-like behavior in Mn: NBT single crystals is strongly related to the tetragonal phase. The tetragonal platelets oriented along three principle directions of the pseudo-cubic cell with the typical thickness of 1–2-unit cells were visualized at the atomic scale. Special AFE-like dipoles were formed by two antiparallel-polarized tetragonal platelets with a thickness of 2-unit cells, which are separated by a thin non-polar and tilt-free transition layer. Such locally structural components vary in both volume and/or size to a certain extent as changing temperature, chemical composition, and external electric field, phenomenologically showing AFE characteristics. Our study provides the decisive atomic-scale information of tetragonal platelets and establishes a hitherto unobserved AFE-like structure existing in the NBT-based materials.

Enhanced Energy Storage Performance in Na0.5Bi0.5TiO3-Based Relaxor Ferroelectric Ceramics via Compositional Tailoring.

Owing to the high power density, excellent operational stability and fast charge/discharge rate, and environmental friendliness, the lead-free Na0.5Bi0.5TiO3 (NBT)-based relaxor ferroelectrics exhibit great potential in pulsed power capacitors. Herein, novel lead-free (1−x)(0.7Na0.5Bi0.5TiO3-0.3Sr0.7Bi0.2TiO3)-xBi(Mg0.5Zr0.5)O3 (NBT-SBT-xBMZ) relaxor ferroelectric ceramics were successfully fabricated using a solid-state reaction method and designed via compositional tailoring. The microstructure, dielectric properties, ferroelectric properties, and energy storage performance were investigated. The results indicate that appropriate Bi(Mg0.5Zr0.5)O3 content can effectively enhance the relaxor ferroelectric characteristics and improve the dielectric breakdown strength by forming fine grain sizes and diminishing oxygen vacancy concentrations. Therefore, the optimal Wrec of 6.75 J/cm3 and a η of 79.44% were simultaneously obtained in NBT-SBT-0.15BMZ at 20 °C and 385 kV/cm. Meanwhile, thermal stability (20–180 °C) and frequency stability (1–200 Hz) associated with the ultrafast discharge time of ~49.1 ns were also procured in the same composition, providing a promising material system for applications in power pulse devices.

Microstructural features and complex electromechanical parameters of lead-free ferroelectric ceramics

An experimental study of the microstructural features and complex electromechanical parameters of lead-free ferroelectric ceramics based on solid solutions of the sodium-lithium niobate system was carried out. The phase distribution, as well as grain and domain structure were studied by piezoresponse force microscopy and scanning electron microscopy. A complete set of complex elastic, dielectric, and piezoelectric parameters of lead-free ferroelectric ceramics Na0.87Li0.13Sr0.01Nb0.99O3 + 1 at.% SiO2 was measured and analyzed using the Piezoelectric Resonance Analysis Program automatic iterative method. The frequency dependences of complex electromechanical parameters of the samples were measured in the frequency range up to 90 MHz.

Effect of Ba2+ ion on the structural, morphological and electrical properties of lead-free Na0.5Bi0.5TiO3 ceramics

Effect of Ba2+ ion on the structural, morphological and electrical properties of lead-free Na0.5Bi0.5TiO3 ceramics

(1-x)(Na0.5Bi0.5)TiO3-x(K0.5Bi0.5)TiO3 ceramics near morphotropic phase boundary: A structural and electrical study

In this work, lead-free (1-x)(Na0.5Bi0.5)TiO3-x(K0.5Bi0.5)TiO3 ferroelectric ceramics with the compositions (x(%)= 0; 12; 16; 17; 18; 19; 20; 30 and KBT) were obtained by solid state reaction at a particularly optimized calcination temperature (1000 °C/4H). A pure perovskite structure was observed for the different compositions. Effect of KBT addition on the microstructure, dielectric, ferroelectric and electrical properties at high temperature of (1-x)NBT-xKBT ceramics are studied using SEM, impedance analyzer and electrical characterization. X-ray diffraction and Rietveld refinement analysis of all samples reveal the presence of a biphasic region in the solid-solution series with a truly rhombohedral-tetragonal symmetry observed only at (x(%) = 16–20). Hence, Raman measurements allow us to conclude that the introduction of K+ creates a local disorder in the structure with a modification of the long-range symmetry. The temperature-dependent dielectric behavior reveals particularly large anomalies in the case of (x = 16%) due to the presence of more local disorder. The ferroelectric-antiferroelectric phase transition mechanisms are discussed to explain the nature of these anomalies. The results suggest a diffuse, probably second-order phase transition from the ferroelectric to the paraelectric phase, depending on the composition of all samples.

New insights into the piezoelectric, thermodynamic and thermoelectric properties of lead-free ferroelectric perovskite Na0.5Bi0.5TiO3 from Ab initio calculations

Ab initio DFT calculations have been performed to investigate a complete set of structural, electronic, elastic, piezoelectric, thermodynamic and thermoelectric properties of lead-free perovskite Na0.5Bi0.5TiO3 (NBT) crystalizes in rhombohedral (R3c), tetragonal (P4bm) and cubic (Pm3̅m) phases. The lattice stabilities in Na0.5Bi0.5TiO3 crystals are studied from the formation energy and phonon dispersions. Electronic band profiles obtained using new KTBmBJ+so potential reveal that R3c, P4bm and Pm3̅m phases of NBT are indirect band gap semiconductors with energy values of ~ 3.29, 3.05 and 3.09 eV, respectively. Analysis of the calculated elastic constants (Cij) indicates that NBT systems are elastically stable. Ferroelectric tetragonal NBT crystal shows relatively high piezoelectric coefficients [d15 = 101.0 pC/N, d31 = 51.3 pC/N and d33 = 81.1 pC/N], compared to the rhombohedral system [d15 = 96.4 pC/N, d31 = 8.93 pC/N and d33 = 21.2 pC/N and d22 = 21.2 pC/N]. Thermodynamic quantities for rhombohedral NBT compound were calculated using the quasi-harmonic Debye model. Thermoelectricity in terms of electrical conductivity σ/τ, thermal conductivity κ/τ, Seebeck coefficient S, figure of merit ZT and thermo power for NBT crystals were examined using BoltzTraP2 code. P4bm-NBT structure showed a largest magnitude of S ~ 201.42 µV/K (at T = 500 K), following by Pm3̅m ~ 173.6 µV/K (800 K) and R3c ~ 158.6 µV/K (100 K). ZT maximized to high values ~ 2.76 (at 700 K), 2.16 (900 K) and 1.96 (400 K) for P4bm, Pm3̅m and R3c structures, respectively. Na0.5Bi0.5TiO3 (NBT) compounds could be a promising candidate for use in manufacturing high-performance piezoelectric devices and developing high-power thermoelectric generators.