The stability of the polarization state in Na0.5Bi0.5TiO3 (NBT) ceramics has been a long-standing problem for its use in piezoelectric applications at elevated temperatures. It has been generally believed that the polarization state, depolarization temperature, and depolarization process are all linked to the grain size in these materials. In this work, we perform a thorough Piezoresponse Force Microscopy (PFM) study of the NBT ceramic samples with substantially different grain sizes sintered as a function of temperature. As-grown, macroscopically poled, and locally poled samples were investigated focusing on the polarization behavior at depolarization temperature. Switching Spectroscopy PFM (SS-PFM) measurements were conducted as a function of grain size and temperature. No direct correlation is observed between the grain size and the switching parameters in any sample. However, temperature-dependent measurements reveal significant differences that are explained by different concentrations of oxygen vacancies. We rationalized the observed behavior, e.g. apparent stabilization of the locally probed polarization above the depolarization temperature, by accumulation and depletion of oxygen vacancies in the vicinity of the internal boundary of the poled region. Significant asymmetry of the PFM hysteresis loops at elevated temperatures confirms this assumption.

The search for lead-free ferroelectrics, driven by environmental concerns, has continued for decades, yet no material has fully replaced PbZrTiO3-based compositions. Introducing core-shell architectures offers a promising route to enhance functional properties; however, their realization in lead-free systems remains limited. In this work, we present a novel method for obtaining core-shell structures in fully sintered lead-free ferroelectric ceramics by thermally treating two ceramic plates in direct contact-hereafter referred to as the contact method. This approach enabled core-shell formation in BaTiO3, SrTiO3, and Na0.5Bi0.5TiO3 systems through interfacial diffusion. Bi over-stoichiometry in NBT strongly promoted shell formation and enhanced interdiffusion, yielding high chemical contrast with diffusion depths exceeding 300 & micro;m. Furthermore, a fully developed core-shell architecture in BaTiO3 was obtained, resulting in a significant improvement in resistivity. These findings demonstrate the potential of the contact method as a simple and versatile route for tailoring the microstructure and functional properties of lead-free ferroelectric ceramics.

The temperature-frequency dependence of dielectric permittivity in Na0.5Bi0.5TiO3 (NBT) -based compositions displays a diffused, frequency-independent maximum along with a frequency-dependent shoulder below this maximum. This behavior deviates from that of both classical ferroelectrics and conventional relaxor ferroelectrics, and its interpretation is further complicated by challenges in linking it to known structural phase transitions. This study proposes a new interpretation of the dielectric behavior of NBT-based materials through a comparative analysis of temperature-frequency permittivity data in both unpoled and poled NBT samples and 0.95Na0.5Bi0.5TiO3-0.05CaTiO3 solid solution over a broad frequency range (10 Hz-100 MHz). Results reveal that the steep permittivity change between the maximum and shoulder-accompanied by pronounced thermal hysteresis-can be attributed to a phase transition between two non-ferroelectric phases. When this contribution is excluded, the dielectric response aligns with classical relaxor ferroelectric behavior. To reconcile this with other known properties of NBT, the "breathing" model is employed, offering a unified framework for understanding its relaxor-like characteristics.

(1-x)Na0.5Bi0.5TiO3-xSr(1-1.5y)Bi(y)TiO(3) solid solutions attract increased interest as lead-free ferroelectrics perspective for sensors, actuators, and energy storage applications. However, thick films of this composition are of great demand, as they could serve as a good compromise between the bulk ceramics and thin films-due to the advantage in realization of miniaturized devices without reducing the power and sensitivity of target devices, as in the case of thin films, and ability to accommodate application of much higher electric fields compared to bulk ceramics. In the present research, for the first time, we have produced free-standing 0.9Na(0.5)Bi(0.5)TiO(3)-0.1Sr(0.7)Bi(0.2)TiO(3) thick films by water-based tape-casting method, using just two organic chemicals, which is an eco-friendly production approach having only several successful attempts in the case of ferroelectric materials before. We conducted a detailed study focusing on development of microstructure at various sintering temperatures and consequences of evaporation of Bi during the production. Our findings show how the choice of the sintering temperature can help in improvement of density of the thick films, reaching 98.9%, and changing the grain size. It is demonstrated that a secondary phase appears predominantly on the free surface of the films. We propose a method for effective minimization of its formation-by the choice of appropriate embedding powder media during the sintering.

Morphotropic phase boundary of ferroelectric solid solutions attracts interest with respect to not only improved piezoelectric properties but also the electrocaloric effect, which is attractive to build new-generation cooling devices. In the present study, the electrocaloric effect in (1 − x − y)Na0.5Bi0.5TiO3–xBaTiO3–yNaNbO3 near its morphotropic phase boundary is studied by direct measurements of temperature change ΔT. ΔT maximum is observed in the region of depolarization temperature, where the major contribution comes from an entropy jump at the electric field-induced phase transition. Differences between ΔT values measured when an electric field pulse is applied and removed are explained by slow domain rearrangement in the ferroelectric phase and metastability of phases close to the depolarization temperature. Among studied compositions, the highest value of ΔT is obtained for the compositions 0.94Na0.5Bi0.5TiO3–0.06BaTiO3 and 0.93Na0.5Bi0.5TiO3–0.06BaTiO3–0.01NaNbO3 in the region of depolarization temperature. Examples of the comparison of the directly measured values of ΔT and the values calculated using the Maxwell relation revealing inconsistency between the two methods in the ferroelectric and the nonferroelectric phase are presented.

Recently, Na0.5Bi0.5TiO3 and its solid solutions are receiving intensive study as one of the most perspective lead-free ferroelectrics. Not only physical properties, but also the structure and nature of phase transitions of these compositions are of great interest, as their previous studies contain many uncertainties. In the present research, Na0.5Bi0.5TiO3 and 0.95Na0.5Bi0.5TiO3–0.05CaTiO3 solid solutions were thoroughly studied focusing on the elastic and thermal expansion characteristics, accompanying the obtained results by x-ray diffraction, scanning electron microscopy, differential calorimetry, and second harmonic generation measurements. Temperature-frequency dependences of dielectric permittivity were observed to be similar for both compositions. In spite of this, the experimentally obtained temperature dependences of thermal expansion and Young's modulus in Na0.5Bi0.5TiO3 and 0.95Na0.5Bi0.5TiO3–0.05CaTiO3 reveal unambiguous differences in the temperature range of the observed or expected (as in the case of Na0.5Bi0.5TiO3) phase transitions. X-ray diffraction patterns are fitted using Pnma symmetry. This allows us to distinguish the temperature regions with different behaviors of lattice parameters, which correlate with the observed behavior of thermal expansion and Young's modulus. A reduction in the intensity of second optical harmonic was observed upon increasing the temperature in the whole studied temperature range. This encourages us to reconsider the mechanism responsible for the temperature dependence of dielectric permittivity.

In the present work, Na0.5Bi0.5TiO3 (NBT) ceramics with the addition of excess Bi in two different ways - before calcination and before sintering - are considered, revealing how the excess Bi affects their microstructure and chemical content. Average grain size is seen to decrease, with the grain size distribution becoming less diffused at higher excess Bi concentrations. The reason for such a feature is the shift of the sintering temperature region where the abnormal grain growth starts to contribute towards higher temperatures. The influence of excess Bi is more pronounced in the case if it is added before calcination. It was discovered that a small amount of excess Bi helps to prevent the formation of Bi-deficient inclusions. While, high concentrations of excess Bi induce the formation of Bi-rich inclusions - most probably Na0.5Bi4.5Ti4O15. Possible mechanisms of formation of both types of inclusions are discussed in detail. Instead of Bi over-stoichiometry, elevated Na content and slightly lower O content were detected in the matrix grains of the sintered NBT ceramics prepared with excess Bi. These deviations increase upon increasing the added excess Bi concentration. The presence of another, Na-rich phase, is assumed, which could not be detected by X-ray diffraction or by energy-dispersive X-ray analysis.

Thermal etching is a widely accepted surface treatment method for studying microstructure in Na0.5Bi0.5TiO3-based compositions. Surprisingly, besides the flat pattern of grains (suitable for evaluating ceramics’ microstructure), images illustrating well-expressed relief and even microstructure consisting of partly bonded cubic-shaped grains are also found among the micrographs presented in various publications. The present paper shows that this different surface character in Eu-modified Na0.5Bi0.5TiO3 can be obtained through thermal treatment across a wide range of temperatures. At higher temperatures, remarkable growth of cubic-shaped grains on the surface is observed. This growth affects the grain size distribution on the surface more than it does within the bulk of a sample. Such micrographs cannot be used to characterise the microstructure of dense ceramics. Intensive growth of TiO2 inclusions at high thermal treatment temperatures is also observed, revealing substantial evaporation of Bi and Na from the surface of a ceramic sample, but not from its core part.

Na0.5Bi0.5TiO3 (NBT)-based compositions are among the most promising lead-free ferroelectrics. Although studies of modification of NBT-based compositions' properties are a hot topic, there has been very little attention paid to their characterization from the standpoint of ceramics and the process of producing them. Here, we report on comprehensive analysis of the influence of sintering temperature across a wide temperature range (1020-1240 degrees C) on the complete microstructure and chemical content of NBT ceramics produced by solid state sintering, which is dominating in producing of NBT ceramics. Thorough attention is paid to the grain size distribution, porosity, and inhomogeneity. It is demonstrated how the grain size distribution monotonously becomes more diffuse and the average grain size increases, upon sequential increasing of the sintering temperature. Along with high density (reaching 98% of the theoretical value), two types of pores are observed. Macroscopic pores form at high sintering temperatures along with a small concentration of residual pores remaining after densification of ceramics and observed throughout the whole range of sintering temperatures. Two types of inclusions are detected, corresponding to the chemical compositions NaBiTi6O14 and TiO2 , as inferred from local energy-dispersive X-ray analysis. Non-intentional non-stoichiometry is not detected in the matrix grains of the NBT ceramics, even if sintered at high temperatures. It is inferred that, instead of changing the composition of the matrix grains, Na and Bi volatilization rather influences the porosity and inhomogeneity of NBT. (C) 2021 Elsevier B.V. All rights reserved.

Photoluminescence and optical second harmonic generation in Er-doped 0.4Na(0.5)Bi(0.5)TiO(3)-(0.6-x)SrTiO3-xPbTiO(3)solid solutions is studied. Earlier, it was shown that upon increasing of PbTiO(3)concentration transfer from relaxor to ferroelectric state takes place in these compositions. Here, role of PbTiO(3)and Er concentration on stability of the ferroelectric state is evaluated, taking into account that luminescence, contrary to X-ray diffraction patterns and second harmonic intensity, is sensitive just to local environment around Er(3+)ions. Luminescence spectra at low temperatures are measured in order to detect variations of local environment in comparison with Er-doped Na0.5Bi0.5TiO3.

Thorough studies of electric field-induced strain are presented in 0.4Na(1/2)Bi(1/2)TiO(3)-(0.6-x)SrTiO3-xPbTiO(3) (NBT-ST-PT) ternary solid solutions. The increase of concentration of lead x induces crossover from relaxor to ferroelectric. Strain in a relaxor state can be described by electrostrictive behavior. The electrostrictive coefficients correspond to other well-known relaxor ferroelectrics. The concentration region with a stable ferroelectric phase revealed that the polarization dependence of strain does not exhibit nonlinearity, although they are inherent to the electric field dependence of strain. In this case, electric field dependence of strain is described in terms of the Rayleigh law and the role of domain wall contribution is extracted. Finally, the character of strain at the electric field-induced phase transition between the nonpolar and the ferroelectric states is studied. The data shows that in the vicinity of the electric field induced phase transition the strain vs. electric field displays electrostrictive character.

The electrocaloric effect as a function of Ca concentration is studied in 0.4Na1/2Bi1/2TiO3-(0.4-x)SrTiO3-0.2PbTiO3-xCaTiO3. Weak influence of Ca at low concentrations corresponds to the previously established concentration dependence of phase transition temperature and polarization, whereas the decrease of electrocaloric effect at phase transition at higher concentrations of Ca can be explained by assuming increasing concentration of polar nanoregions. The role of thermal depoling in the case of electric-field-induced phase transition is shown.

In this paper we present our measurements of the linear and nonlinear dielectric permittivity of 0.4Na0.5Bi0.5TiO3–(0.6-x)SrTiO3–xPbTiO3 solid solutions (x=0, 0.05, 0.1, 0.15). The dielectric anomaly increases in the system with respect to the concentration of lead, showing that interactions between dipolar entities are modified. The system exhibits dipolar-glass-like behaviour at low values of x (0⩽x<0.1). Relaxor behaviour emerges in the sample where x=0.1. Furthermore, a spontaneous first-order phase transition from relaxor to normal ferroelectric is observed at x⩾0.15. A few peculiar dispersion regions are observed in the ferroelectric phase, which we attribute to the coexistence of ferroelectric and dipolar glass phases. On the other hand, the nonlinear dielectric response indicates the existence of compressible polar nanoregions as common elementary dipolar entities in both relaxor and glass-like states. Dielectric spectra were approximated with empirical Cole–Cole or Havriliak–Negami equations. It is shown that the temperature dependencies of mean relaxation times, which were obtained from dielectric spectra, follow the Vogel–Fulcher equation in all samples. Moreover, distributions of relaxation times are obtained and a phase diagram is presented.

We present dynamic mechanical analysis (DMA) and thermomechanical analysis (TMA) measurements of a new type of polyurea elastomer nanocomposites based on inorganic MoS2 nanotubes and Mo6S2I8 nanowires. The addition of a small amount of nanoparticles (<1 wt-%) leads to an increase of the glass transition temperature Tg as compared to the pure elastomeric matrix. A second peak observed in tand in the pure and mixed elastomer is attributed to a second glass transition occurring in regions near the hard nanodomains of the microphase separated polyurea system. It is also found that the small amount of nanoparticles leads to an increase in the Young´s modulus of up to 15 % in the whole measured temperature range (from -130 °C to 20 °C). The thermal expansion of doped samples is considerably larger above Tg. Below Tg, this difference vanishes completely. A very similar behaviour was also found in measurements of polyisoprene/multiwall carbon nanotube (MWCNT) composites.

Continuous transfer from relaxor to normal ferroelectric phase transition is found in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3 if concentration of PbTiO3 increases. The relaxor properties in NBT solid solutions are described by the power law and Vogel-Fulcher relationship, and concentration dependence of the relevant parameters is discussed. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim