Structural,ferroelectric, and optical properties of Pr-NBT-xCTO relaxor ferroelectric thin films

Sol-gel method was used to prepare the Pr³⁺ ions-doped (1-x)Na_0.5Bi_0.5TiO₃-xCaTiO₃ (Pr-NBT-xCTO) (x?=?0, 0.04, 0.06, 0.08, 0.1, 0.12, and 0.16) thin films on Pt/Ti/SiO₂/Si and fused silicon substrates. The structure phase of thin films was evolving from rhombohedral (R3c) to orthorhombic (Pnma) with increasing CTO content. Owing to the morphotropic phase boundary (MPB), the improved ferroelectric and dielectric properties were obtained at x?=?0.06–0.1. The MPB was formed from the concomitant phase of rhombohedral (R3c) and orthorhombic (Pnma). The Pr-NBT-0.08CTO thin film showed the best ferroelectric and dielectric properties, as well as strong relaxor behavior (the diffusion factor is γ?=?1.79). In addition, all the films exhibited strong red emission as excited by UV light, and wide optical band-gap (3.44–3.47?eV), which might be influenced by grain size and structural variation. Our results indicate that Pr-NBT-xCTO thin films may have potential applications in ferroelectric-luminescence multifunctional optoelectronic devices.     

Relaxation behavior of PbZrO3-SrTiO3 thin film for enhancing energy storage performances

In recent years, antiferroelectric materials have attracted significant attention as energy storage materials in pulsed power systems. In this study, (1-x)PbZrO₃-xSrTiO₃ (PZO-STO) antiferroelectric films were prepared, and the effects of the STO content on the microstructure and energy storage performance of the thin films were investigated in detail. The results showed that when the PZO/STO ratio was near the morphotropic phase boundary, the long-range PZO-STO-ordered structure could be broken by the paraelectric nanograins generated at the grain boundary. The number of nanoparticles increased gradually with an increase in the STO content, thereby leading to the microstructure transformation of the thin films from antiferroelectric to relaxation ferroelectric. When the STO content was 20%, the as-prepared thin film had a maximum energy storage density of 15.26 J/cm³, which was 117.14% higher than that of the pure PZO thin film.     

Structure,microstructure, dielectric and piezoelectric properties of (1-x-y)BiFeO3-xPbFe0.5Nb0.5O3-yPbTiO3 ceramics

Ceramics of seven quasi-binary concentration sections of the ternary solid solution system (1-x-y)BiFeO₃-xPbFe_0.5Nb_0.5O₃-yPbTiO₃ were prepared by the conventional solid-phase reaction method in the range of 0.05 ≤ x ≤ 0.325; 0.05 ≤ y ≤ 0.325. By using x-ray diffraction technique, the phase diagram of the system was constructed which was shown to contain the regions of tetragonal and rhombohedral symmetry and the morphotropic phase boundary between them. Grain morphology, dielectric and piezoelectric properties of selected solid solutions were investigated. The highest piezoelectric coefficient d₃₃ = 50 pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior and region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 600–800 K.     

Polarization behavior of` lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films with enhanced ferroelectric properties

(Bi_0.5Na_0.5)TiO₃ based ferroelectric lead-free thin films have great potential for modern micro-devices. However, the multicomponent feature and volatile nature of Bi/Na makes the achievement of high quality films challenging. In this work, the morphotropic phase boundary composition, 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ thin films were successfully prepared by CSD method. Dense films with low dielectric loss and low leakage current density were obtained. A well-defined polarization hysteresis loop with a high remnant polarization was observed in the thin films. Moreover, the polarization behavior of the film at original state, under electric field and upon heating was investigated by PFM. A self-polarization and asymmetric domain switching behavior were observed. High temperature induced depolarization and the self-polarization recovered upon cooling. The thin films with good quality show a promising potential for the application in electrical devices, and the in-depth investigation of the polarization behavior improves the understanding of ferroelectric and piezoelectric properties of thin films.     

High-recoverable energy-storage density and dielectric tunability in Eu3+-doped NBT-xSTO binary solid solution films

The Eu³⁺-doped (1 − x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (Eu-NBT-xSTO) thin films were prepared on Pt/Ti/SiO₂/Si substrates. Raman analysis reveals that the phase structure may undergo a phase evolution of rhombohedral → rhombohedral + tetragonal (morphotropic phase boundary) → tetragonal with increasing content of STO. The scanning electron microscopy images show that the uniformity and high density of Eu-NBT-xSTO films were increased by adding STO, resulting in a pronounced effect on energy storage properties. The ɛ-T curves confirm that a high phase transition diffuseness of γ = 2.02 ± 0.03 and 1.98 ± 0.03 was achieved in Eu-NBT-0.24STO and Eu-NBT-0.3STO films, respectively. Furthermore, a large recoverable energy storage density of 31.5 J cm⁻³ with an efficiency of 64% was obtained in Eu-NBT-0.3STO film, which also exhibited good thermal stability in the temperature range between −60°C and 80°C as well as long-term stability up to 1 × 10⁸ switching cycles. These results suggest that the Eu-NBT-xSTO films may be used in the novel and advanced energy storage capacitors.     

Normal-relaxor ferroelectric phase transition induced morphotropic phase boundary accompanied by enhanced piezoelectric and electrostrain properties in strontium modulated Bi0.5K0.5TiO3 lead-free ceramics

In this work, (Bi_0.5K_0.5)_1-xSr_xTiO₃ compositions (x = 0.03∼0.18) are designed to clarify the role of normal-relaxor ferroelectric phase transition and morphotropic phase boundary on dielectric, piezoelectric and electrostrain properties. With increasing strontium content, tetragonal distortion decreases and tetragonal and pseudocubic phases coexist in 0.09 ≤ x ≤ 0.15 compositions; the spontaneous phase-transition temperature and curie temperature decrease, as certified by phase-structure, dielectric properties and Raman spectra analysis. Optimized piezoelectric constant ∼106 pC/N and electrostrain ∼0.17 % are obtained for (Bi_0.5K_0.5)_0.88Sr_0.12TiO₃ composition. Piezoelectric force microscopic technique is exploited to clarify the origin of enhancement in macroscopic performances. Increase in temperature enhances ferroelectric performance and a large strain value ∼0.25 % with low hysteresis ∼27 % are obtained at 140 °C for the optimized composition, which are believed to originate from electric-field induced relaxor-to-ferroelectric phase transition with thermally-activated reduced energy barriers. This work clearly demonstrates that lead-free Bi_0.5K_0.5TiO₃-based ceramics are another promising bismuth-based species in applications of piezoelectric sensors and actuators.     

Structure,dielectric and ferroelectric properties of 0.92Na0.5Bi0.5TiO3–0.06BaTiO3–0.02K0.5Na0.5NbO3 lead-free ceramics: Effect of Co2O3 additive

0.92Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–0.02K_0.5Na_0.5NbO₃+x wt% Co₂O₃ (NBKT–xCo, x=0, 0.2, 0.4, 0.6, 0.8) lead-free ferroelectric ceramics were prepared via a conventional solid state reaction method. Effects of Co₂O₃ additive on crystallite structure, microstructure, dielectric and ferroelectric properties of the NBKT–xCo ceramics were studied. X-ray diffraction results showed that the rhombohedral–tetragonal morphotropic phase boundary existed in all the ceramics, with relative amount of tetragonal phase varying with the content of Co₂O₃. Average grain size, maximum value of dielectric constant, Curie temperature and ferroelectric properties of the ceramics were close related to the content of Co₂O₃. The dielectric anomaly caused by the phase transition between the ferroelectric phase and the so-called “intermediate phase” was observed in the ceramics with x≤0.2, while it disappeared with further increasing x. All the ceramics showed a diffuse phase transition between the “intermediate phase” and the paraelectric phase. The change in the ferroelectric properties with changing the content of Co₂O₃ was discussed by considering the competitive effects among grain size, relative amount of the tetragonal phase and oxygen vacancies.     

Effect of BiMeO3 on the Phase Structure,Ferroelectric Stability,and Properties of Lead‐Free Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics

The effects of BiMeO₃ (Me = Fe, Sc, Mn, Al) addition on the phase transition and electrical properties of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT20) lead‐free piezoceramics were systematically investigated. Results showed that addition of BiFeO₃ into BNKT20 induces a phase transition from tetragonal–rhombohedral coexisted phases to a tetragonal phase with the observation of enhanced piezoelectric properties (d₃₃ = 150 pC/N for 0.02BiFeO₃). BiScO₃, BiMnO₃, and BiAlO₃ substitutions into BNKT20 induce a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point T_F–R to room temperature. Accordingly, large accompanying normalized strains of 0.34%–0.36% are obtained near the ferroelectric–relaxor phase boundary, and the mergence of large strain response can be ascribed to a reversible field‐induced ergodic relaxor‐to‐ferroelectric phase transformation. Moreover, our study also revealed that the composition located at the ferroelectric–relaxor phase boundary where the strain response is consistently derivable shifts to a BNKT20‐rich composition as the tolerance factor t of the end‐member BiMeO₃ increases, and this relationship is expected to provide a guideline for designing high‐performance (Bi_0.5Na_0.5)TiO₃‐based materials by searching the ferroelectric–relaxor phase boundary.     

Piezoelectric properties and phase transition temperatures of the solid solution of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3

The phase diagram of (1 ? x)(Bi_0.5Na_0.5)TiO₃–xSrTiO₃ was completed and investigations on polarization and strain in this system were carried out. (1 ? x)(Bi_0.5Na_0.5)TiO₃–xSrTiO₃-ceramics were prepared by conventional mixed oxide processing. The depolarization temperature (T_d), the temperature of the rhombohedral–tetragonal phase transition (T_r–t) and the Curie temperature (T_m) were determined by measuring the temperature dependence of the relative permittivity. All solid solutions of (1 ? x)(Bi_0.5Na_0.5)TiO₃–xSrTiO₃ show relaxor behavior (A-site relaxor). From XRD-measurements a broad maximum of the lattice parameter can be observed around x = 0.5 but no structural evidence for a morphotropic phase boundary was found. SEM-analysis revealed a decrease of the grain size for increasing SrTiO₃-content. At room temperature a maximum of strain of about 0.29% was found at x = 0.25 which coincides with a transition from a ferroelectric to an antiferroelectric phase. The temperature dependence of the displacement indicates an additional contribution from a structural transition (rhombohedral–tetragonal), which would be of certain relevance for the existence of a morphotropic phase boundary.     

Phase formation,electrical properties and morphotropic phase boundary of 0.95Pb(ZrxTi1−x)O3–0.05Pb(Mn1/3Nb2/3)O3 ceramics

Ferroelectric ceramics in specific composition of 0.95Pb(Zr_xTi_1?x)O₃–0.05Pb(Mn_1/3Nb_2/3)O₃ or PZT–PMnN (with x=0.46, 0.48, 0.50, 0.52, and 0.54) have been investigated in order to identify the morphotropic phase boundary (MPB) composition. The effects of Zr/Ti ratio on phase formation, dielectric and ferroelectric properties of the specimens have also been investigated and discussed. X-ray diffraction patterns indicate that the MPB of the tetragonal and rhombohedral phase lies in x=0.52. The crystal structure of PZT–PMnN appeared to change gradually from tetragonal to rhombohedral phase with increasing Zr content. The dielectric and ferroelectric properties measurements also show a maximum value (ε_r, tan δ and P_r) at Zr/Ti=52/48, while the transition temperature decreases with increasing Zr content.     

Actuation mechanisms in mixed-phase K0.5Bi0.5TiO3-BiFeO3-PbTiO3 ceramics

We report an in-situ synchrotron X-ray diffraction study of K_0.5Bi_0.5TiO₃-BiFeO₃-PbTiO₃ ceramics, which exhibit a T_c of around 450 °C. The electromechanical actuation mechanisms comprise contributions from coexisting tetragonal and rhombohedral phases. The tetragonal {200} grain family exhibited the highest effective lattice strain, up to 8.2 × 10⁻³ at 5 kV/mm. Strong strain anisotropy in the tetragonal phase and field-induced intergranular stresses facilitate a partial transformation from tetragonal (high strain anisotropy) to rhombohedral (low strain anisotropy) at high electric field levels, with an average linear transformation strain of -1.54 × 10^-3. The domain switching behavior was effectively enhanced in both tetragonal and rhombohedral phases after the phase transformation, due to the release of intergranular stress. This observed self-adapting mechanism in tuning intergranular stress through partial phase switching in the morphotropic KBT-BF-PT composition with large lattice distortion could also be exploited in other perovskite systems in order to achieve high performance high temperature piezoelectric ceramics.     

Morphotropic Phase Boundary in Solution‐Derived (Bi0.5Na0.5)1−xBaxTiO3 Thin Films: Part I Crystalline Structure and Compositional Depth Profile

In this work, ferroelectric (Bi_0.5Na_0.5)_1?xBa_xTiO₃ thin films were fabricated by chemical solution deposition (CSD) with compositions x = 0.050–0.150. Stoichiometric thin films (hereinafter BNBT) and others containing 10 mol% excesses of Bi³⁺ and Na⁺ (BNBTxs) were spin coated onto Pt/TiO₂/SiO₂/(100)Si substrates and crystallized by rapid thermal processing at 650°C for 60 s in oxygen atmosphere. Crystalline structure is studied by X‐ray diffraction using Cu anode (λ_Cu = 1.5406 ?) and synchrotron radiation (λ = 0.97354 ?). Rietveld refinement showed the coexistence of rhombohedral/tetragonal phases in the BNBT films for x values close to those reported for (Bi_0.5Na_0.5)_1?xBa_xTiO₃ bulk ceramics. Different volume fractions of the rhombohedral/tetragonal phases are detected as a function of the Ba²⁺ content. An apparent shift of the position of the morphotropic phase boundary (MPB) is observed in the BNBTxs films. Here, the MPB region appears for nominal Ba²⁺ molar values of x ~ 0.10 and the experiments using a grazing‐incidence synchrotron radiation indicate the existence of a crystalline phase with pyrochlore structure at the film surface. Rutherford backscattering experiments (RBS) revealed that the bismuth excess is not volatilized during the crystallization of the BNBTxs films which present inhomogeneous compositional depth profile and thick Bi_xPt bottom interfaces. The MPB BNBT films with x ~ 0.055 have a homogeneous compositional depth profile without appreciable bottom interfaces. Scanning electron micrographs reveal less porosity and higher grain sizes in the stoichiometric films than in those with Bi³⁺ and Na⁺ excesses.     

Ferroelectric and magnetic properties in (1−x)BiFeO3–x(0.5CaTiO3–0.5SmFeO3) ceramics

Multiferroic ceramics were prepared and characterized in (1?x)BiFeO₃–x(0.5CaTiO₃–0.5SmFeO₃) system by a standard solid‐state reaction process. The structure evolution was investigated by X‐ray diffraction and Raman spectrum analyses. The refinement results confirmed the different phase assemblages with varying amounts of polar rhombohedral R3c and nonpolar orthorhombic Pbnm as a function of the substitution content. In the compositions range of 0.2≤x≤0.5, polar R3c and nonpolar Pbnm coexisted, which was referred to polar‐to‐nonpolar morphotropic phase boundary (MPB). According to the dielectric and DSC analysis results, the ceramics with x≤0.2 changed to diffused ferroelectric, and the ferroelectric properties were enhanced significantly. Two dielectric relaxations were detected in the temperature range of 200‐300 K and 500‐700 K, respectively. The high‐temperature dielectric relaxation was attributed to the grain‐boundary effects. While the low temperature dielectric relaxation obtained in the samples with x=0.3‐0.5 was related to the charge transfer between Fe²⁺ and Fe³⁺. The magnetic hysteresis loops measured at different temperature indicated the enhanced magnetic properties in the present ceramics, which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions. In addition, the rare‐earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.     

Phase transition and energy storage properties of Bi0.5Na0.5TiO3–Bi(Mg2/3Nb1/3)O3 lead-free ceramics

Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics have been extensively studied due to their excellent dielectric, piezoelectric and ferroelectric properties. The phase structure and functionalities of BNT can be feasibly adjusted by doping/forming solid solutions with other elements/components. In this work, Bi(Mg_2/3Nb_1/3)O₃ (BMN) was introduced into BNT by a conventional solid-state reaction to form a homogeneous solid solution of (1-x)(Bi_0.5Na_0.5)TiO₃-xBi(Mg_2/3Nb_1/3)O₃ (BNT-xBMN) with a perovskite structure. With the increase of BMN content, a phase transition from rhombohedral R3c to tetragonal P4bm has been confirmed by XRD, along with shifting the ferroelectric-paraelectric phase transition temperature to lower temperatures with broadening dielectric peaks. Furthermore, an optimized recoverable energy density of 1.405 J/cm³ was achieved for BNT-0.10BMN ceramics under a low applied electric field of 140 kV/cm, which is mainly attributed to the transformation from ferroelectric to ergodic relaxor phase.     

Morphotropic Phase Boundary in Solution‐Derived (Bi0.5Na0.5)1−xBaxTiO3 Thin Films: Part II Functional Properties and Phase Stability

The analysis of the functional properties (ferroelectric, dielectric, and piezoelectric) of chemical solution deposited thin films of the lead‐free (Bi_0.5Na_0.5)_1?xBa_xTiO₃ (BNBT) solid solution prepared from solution precursors with and without Na⁺ and Bi³⁺ excesses has been performed in this work. At room temperature a nonergodic relaxor ferroelectric state has been found. The switched polarization of the films is not stable at room temperature, poor remnant polarization, associated with an enhancement of the induced domains randomization produced by the films constraints. The depolarization temperature for the switched polarization allowed us to build up a tentative phase diagram for these BNBT films. Both the better functional properties and the agreement of the depolarization temperature with the freezing temperature of the relaxor Volger–Fulcher behavior permit to locate the center of the morphotropic phase boundary region close to x = 0.055 in the stoichiometric films and x = 0.10 for the films with Na⁺ and Bi³⁺ excesses. Based on these results, the possible applications of these films are discussed.     

Effect of Li addition on phase formation behavior and electrical properties of (K0.5Na0.5)NbO3 lead free ceramics

In this work, Li-modified KNN ceramic compositions ((K_0.5Na_0.5)_1−xLi_x)NbO₃ with x = 0.03, 0.04, 0.05, 0.06, 0.65 and 0.07 were prepared by a conventional solid-state mixed-oxide method. The structural phase formation and microstructure were characterized by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM). It has been found that a morphotropic phase boundary (MPB) between orthorhombic phase and tetragonal phases should exist between compositions with Li contents of 6-6.5%. The Curie temperature (T_c) of the ceramics shifted to higher temperature with increasing Li content. The room temperature dielectric constant was also seen to be higher than the pure KNN ceramics. In addition, the ferroelectric properties were found to enhance at near MPB compositions. This study clearly showed that the addition of Li could improve the dielectric and ferroelectric properties in (K_0.5Na_0.5)NbO₃ ceramics.     

Structure and ferroelectric behaviour of Na0.5Bi0.5TiO3-KNbO3 ceramics

Lead-free piezoelectric ceramics, (1?x)Na_0.5Bi_0.5TiO₃-xKNbO₃ (NBT-xKN), with x?=?0.02–0.08 were fabricated by solid-state reaction and sintering. The crystal structures and dielectric properties were measured for different KN contents. All compositions in the unpoled, as-sintered state were found to be single-phase pseudo-cubic. However, typical ferroelectric behaviour, with well-saturated polarisation-electric field hysteresis loops, was observed for certain compositions at high electric field levels. It is shown using high-energy synchrotron X-ray diffraction that the application of the electric field induced an irreversible structural transformation from the nano-polar pseudo-cubic phase to a ferroelectric rhombohedral phase. The changes in lattice elastic strain and crystallographic texture of a poled NBT-0.02KN specimen as a function of the grain orientation, ψ, conform well to those expected for a conventional rhombohedrally distorted perovskite ferroelectric ceramic. The dielectric permittivity-temperature relationships for all compositions exhibit two transition temperatures and a frequency-dependent behaviour that is typical of a relaxor ferroelectric. The transition temperatures and grain size decrease with the increasing KN content.     

Electrical properties and relaxor phase evolution of Nb-Modified Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-SrTiO3 lead-free ceramics

In this work, the crystalline phase, domain structure, and electrical properties of [Bi_0.5(Na_0.84K_0.16)_0.5]_0.96Sr_0.04Ti_1-xNb_xO₃ (x = 0.010–0.030) ceramics are investigated. Increasing the Nb content induces the phase transition from coexistent rhombohedral and tetragonal phases to a single pseudo-cubic phase, and the lamellar ferroelectric domains evolve into polar nanoregions. Decreased ferroelectric-to-relaxor transition temperature and enhanced frequency dispersion are found in the temperature-dependent dielectric constant and loss, implying a transition from the non-ergodic to ergodic relaxor state. The Nb substitution significantly degrades the long-range ferroelectric order with sharply decreased piezoelectric coefficients from ? 140 to ? 1 pC/N. However, a large strain of 0.32% at 5 kV/mm (normalized strain of 640 pm/V) is obtained around the critical composition of x = 0.0225. The composition of x = 0.030 shows good temperature insensitivity of the strain response, characterized by 308 pm/V with less than 15% reduction from 25 °C to 125 °C.     

Characterization of Bi0.5Na0.5TiO3-PbTiO3M Dielectric Materials

Bi_0.5Na_0.5TiO₃ (BNT) modified with Pb showed an increased and broadened dielectric constant and limited grain growth. Pb also lowered the first transition temperature. The phases of BNT were confirmed to be ferroelectric at room temperature which transformed to antiferroelectric above 220°C. When BNT was doped with 10% Pb, the first transition decreased to 140°C and abruptly disappeared in the composition with 17% Pb. The crystal structure of 17%-Pb-doped BNT at room temperature is tetragonal, which differs from the rhombohedral structure at lower Pb contents. Thus, the phase boundary between rhombohedral and tetragonal ferroelectric phase was determined to be between 15% to 17% Pb in this study.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.