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Solid-state refrigeration based on electrocaloric effect (ECE) is regarded as one of the potential candidates to replace the vapor-compression refrigeration. Nowadays, the challenge for ECE refrigeration devices is mainly due to the lack of high-performance ECE materials. Recently, the lead-free NaNbO₃(NN)-based ceramics have attracted great interest in ECE properties due to their multi-phase structures, which may generate large quantities of polar states. In this work, the ($1-x$)[0.9NaNbO${}_3-0.10$Ba(Zr${}_{0.15}{\mathrm{\text{Ti}}}_{0.85}){\mathrm{\text{O}}}_3]-x$ Bi(${\mathrm{\text{Mg}}}_{0.5}{\mathrm{\text{Zr}}}_{0.5}){\text{O}}_3$ (NN-BZT-BMZ, $x=0$, 0.02, 0.04 and 0.06) ceramics were designed via adjusting the ratio of relaxor ferroelectric phase (RFE) over normal ferroelectric (NFE) phase. X-ray diffraction (XRD) patterns indicate the phase changes from ferroelectric (FE) tetragonal phase to pseudo-cubic phase with the increasing Bi(Mg${}_{0.5}$Zr${}_{0.5})$O₃ (BMZ) content. Raman spectroscopy and piezoelectric force microscopy verified the polar nanoregions (PNRs) in sample, indicating the coexistence of NFE and RFE phases. Direct ECE measurement indicated that a $\mathrm{\Delta }T=0.62$K was obtained in $x=0.02$ sample at 5MV/m and 80^∘C via a modified differential scanning calorimetry (DSC) setup. This work provides a novel strategy to design electrocaloric materials with large ECE.

The lead-free ceramics Na${}_{0.42}$Bi${}_{0.42}$Ba${}_{0.06}$TiO${}_{3{-\delta }_{}}$ (BNBT) were prepared via a solid-state sintering method, which was sintered in nitrogen (N₂) and oxygen (O₂) atmospheres, respectively. The photo-dielectric effect (PDE) of BNBT was studied in detail. The light with a wavelength of 450nm and an intensity of 200mW/cm² was used to illuminate the samples. For both ceramics, the dielectric constant varies periodically with changing light on and off, while dielectric loss is almost unaffected by the light irradiation. The dielectric constant of the ceramics in the studied frequency range between 100Hz and 1MHz increases under light illumination, demonstrating obvious PDE. The sintering atmospheres and post-annealing atmospheres affect the increased amplitude of the dielectric constant caused by the light irradiation. The results demonstrate that the PDE of the BNBT ceramics can be regulated by using the processing atmospheres, thus providing guidance for regulating the PDE of lead-free ceramics.

0.8BaTiO₃–0.2Bi_0.5K_0.5TiO₃ (BT–BKT20) lead-free ceramics were prepared by conventional solid state reaction method followed by high energy ball milling. The formation of a single phase tetragonal structure in the material was confirmed by X-ray diffraction. Frequency and temperature-dependent dielectric studies show relaxor behavior in the BT–BKT20 which was found to obey modified Curie–Weiss law with degree of diffuseness 1.573. Complex impedance and electric modulus spectroscopy studies reveal temperature-dependent relaxation process in the material. The Cole–Cole plots were measured at high temperatures at which grain effect was observed. Impedance and electric modulus spectroscopy studies show non-Debye kind of conductivity relaxation process in the present material. Activation energies were calculated from impedance and electric modulus spectroscopy and the values of activation energy indicated that the conduction is ionic in nature. AC and DC conductivity have been measured and studied at different temperatures.

This work reports the influence of sintering temperature on structure, microstructure and piezoelectric properties of 0.48 Ba(Zr${}_{0.2}$Ti${}_{0.8}$)O₃–0.52 (Ba${}_{0.7}$Ca${}_{0.3}$)TiO₃(BZT–BCT) doped with ZnO nanoparticle ceramics manufactured by a conventional solid state reaction method. By increasing sintering temperature, the piezoelectric behaviors were improved and rose up to the best parameters at a sintering temperature of 1450${}^{\circ }$C ($d_{33}=576$ pC/N and $k_p=0.55$). The corresponding properties of undoped BZT–BCT ceramics were investigated as a comparison. The received results show that the sintering behavior and piezo-parameters of doped BZT–BCT samples are better than the undoped BZT–BCT samples at each sintering temperature.

A novel lead-free transparent electro-optic ceramic (K_0.5Na_0.5)_0.95Li_0.05Nb_0.95Bi_0.05O₃+ 2 mol% excess Bi₂O₃ has been fabricated by hot-press sintering. The ceramic possesses a dense, fine-grained and pure-perovskite structure. Owing to the decreases in optical anisotropy of the crystal structure and optical scattering by the fine grains, the ceramic becomes visually clear. It exhibits a high transmittance of 60% in the near-IR region as well as a high effective linear electro-optic coefficient of 102 pm V^-1. Although a relaxor behavior is induced, the ceramic exhibits reasonably good ferroelectric and piezoelectric properties (d₃₃ = 64 pC N^-1).

A new lead-free solid solution of (1–x)NaNbO₃-xBa_0.85Ca_0.15(Ti_0.9Zr_0.1)O₃ was prepared by a traditional sintering method and its phase transition, dielectric and piezoelectric properties were studied. Ba_0.85Ca_0.15(Ti_0.9Zr_0.1)O₃ diffuses into NaNbO₃ lattices to form a new solid solution with perovskite structure. The addition of Ba_0.85Ca_0.15(Ti_0.9Zr_0.1)O₃(x≥0.025) transforms NaNbO₃ from antiferroelectric to ferroelectric. The diffusive ferroelectric–paraelectric phase transition is induced in the ceramics with high concentration of Ba_0.85Ca_0.15(Ti_0.9Zr_0.1)O₃. The ceramics with x = 0.05–0.125 possess large P_r values of 18.6–25.5 μC/cm². A morphotropic phase boundary between tetragonal and orthorhombic phases is formed at 0.05 < x < 0.15, leading to a significant enhancement of piezoelectric properties. The ceramic with x = 0.125 situated near the morphotropic phase boundary exhibits the optimum piezoelectric properties: d₃₃ = 151 pC/N and k_p = 31.6%.

Lead-free multifunctional ceramics of Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃-x mol% Dy have been prepared by an ordinary sintering method and the effects of Dy₂O₃ doping on structure, piezoelectric, ferroelectric and photoluminescent properties of the ceramics have been studied. The ceramics possess a single phase perovskite structure. The grain growth of the ceramics is prohibited and the ferroelectric–paraelectric phase transition at T_C becomes more diffusive after the addition of Dy₂O₃. Dy₂O₃ doping improves the piezoelectricity of the ceramics and the optimal piezoelectric properties d₃₃ = 335 pC/N is obtained at x = 0.5. The addition of 2 mol% Dy enhances the photoluminescent properties of the ceramics and strong emissions at ∼ 478 nm and ∼ 575 nm are observed. Our study shows that the ceramics with low Dy₂O₃ levels exhibit simultaneously the strong piezoelectricity, ferroelectricity and photoluminescence and may have a potential application in mechano-electro-optic integration and coupling device.

Envisaged through adding sintering additives to achieve low-temperature sintering preparation, in order to overcome the volatilization of sodium ions and potassium ions in the high-temperature preparation, so as to improve the density and electrical properties of KNN-based piezoelectric ceramics. This research uses traditional solid-state sintering technology, a high density, high properties lead-free piezoelectric ceramics, KNNSC-$x$, successfully prepared with sintering additives CuO. The modern test analysis of XRD and SEM shows that a moderate amount of CuO doped in the range of research can form a single perovskite structure of an orthorhombic structure, not found in any second phase, and can promote grain evenly growing and improve the sintering properties of KNN-based piezoelectric ceramics. The KNNSC-0.04 ceramics exhibit excellent electrical properties through various electrical tests such as $d_{33}$ = 238pC/N, $k_p$= 47%, ${𝜀}_r$= 1049, tan$\delta$= 2.4%, $P_r$ = 25.6 $\mu$C/cm², $E_C$ = 1.24 kV/mm, respectively. These test results show that the KNNSC-$x$ piezoelectric ceramics have great potential to be applied in middle- and low-voltage piezoelectric devices.

In order to explore the high temperature stability of ceramic capacitor, we present temperature-independent dielectric properties of 0.82[0.94Bi_0.5Na_0.5TiO₃–0.06BaTiO₃]–0.18K_0.5Na_0.5NbO₃ (BNT–BT–18KNN) ceramics. For different sintered temperature and annealing treatment, the pseudoternary system showed a ε_r of 2265 at 1 kHz at 35°C with a normalized permittivity ε/ε_35°C varying less than ±15% from 11°C to 382°C. This pure perovskite phase with slimmer and heat proof P–E loops possessed energy density of 0.616 J/cm³ with a tolerance of about ±3% in a temperature interval ranging from 20°C to 120°C, which is higher and more temperature stable than most ceramic capacitors, such as PLZST and 0.89BNT–0.06BT–0.05KNN. This relaxor ferroelectric (at room temperature) with parabolic bipolar strain–electric (S(E)) curve showed a quite low temperature dependence of positive strain with less than ±6.5% tolerance from the average value of 0.091% between 20°C and 120°C, which is also more temperature stable than the same composition Zhang et al. reported. These merits demonstrate that the newly produced BNT–BT–18KNN ceramics should be a promising candidate for the development of high-temperature capacitor and actuator materials.

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x%Ce₂O₃ + y%La₂O₃ + z%Y₂O₃ compounds (named BNT–BT– x/y/z) were synthesized by a sol–gel technique, for (x,y,z) = (0,0,0), (0.25, 0.25, 0.25), (0.25, 0.5, 0.5), (0.5, 0.25, 0.25) and (0.5, 0.5, 0.25). The structural variation according to the different system compositions is investigated by X-ray diffraction analyses. The results shows that the addition of Ce₂O₃, La₂O₃ or Y₂O₃ in BNT–BT system, do not modify the crystalline structure, and a rhombohedral–tetragonal morphotropic phase boundary is maintained for different dopant addition. The BNT–BT–x/y/z-based ceramics sintered at relatively low temperature (1100°C) exhibit a good densification ratio. The optimum dielectric and piezoelectric properties are obtained with the BNT–BT–0.5/0.25/0.25 composition. High dielectric properties at room temperature (ε_r > 1000) and low dielectric losses (tan δ < 4 × 10^-2) are obtained for this composition. This compound exhibits a piezoelectric constant d₃₃ of 95 pC/N, a good polarization behavior is observed with high remanent polarization P_r of 9.21 μC/cm² and low value coercitive field E_c of 2.66 kV/mm.

(1-x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ (BT-BMT, 0.3 ≤ x ≤ 0.6) ceramics were prepared by the conventional mixed oxide method. Dielectric measurements showed highly diffusive and dispersive relaxor-like characteristics for all compositions. For 0.3 ≤ x ≤ 0.4, all these compositions possess a stable dielectric permittivity of approximately 1500 and low loss tangent (tan δ < 2%) in the temperature range 200–400°C, which may have great potential for high-temperature applications. For all samples, examination of the polarization hysteresis behavior shows an obvious linear hysteresis loops at 120°C. It was found that the energy density of about 0.7 J/cm³ could be achieved at 120°C for x = 0.4 sample, indicating a high energy density.

This work focuses on the structural, electrical and magnetic properties of Bi${}_{0.8}$Tb${}_{0.1}$Ba${}_{0.1}$Fe${}_{0.9}$Ti${}_{0.1}$O₃ ceramics, fabricated by solid state reaction procedure. XRD forms of the samples at RT exhibited perovskite phase through the hexagonal structure at room temperature. Dielectric studies of the materials with frequency at different temperatures (25–400${}^{\circ }$C) exhibit two dielectric anomalies, first at 175${}^{\circ }$C (ferroelectric–ferroelectric transition) and second at around 320${}^{\circ }$C (ferroelectric–paraelectric transition). The Curie temperature moved towards the low side temperature with the increase in frequency. The less value of activation energy got for these samples could be attributed to the influence of electronic contribution to the conductivity. A significant change in the magnetic studies was observed for Bi${}_{0.8}$Tb${}_{0.1}$Ba${}_{0.1}$Fe${}_{0.9}$Ti${}_{0.1}$O₃ ceramic. The impedance analysis confirms the non-Debye type nature of the ceramic and relaxation frequency moved to a higher temperature. The Nyquist plot and conductivity studies showed the NTCR behavior of samples. The highest magnetization field was found at temperature $-$268.15${}^{\circ }$C.

In this work, lead-free systems; ($1-x$) (K${}_{0.465}$Na${}_{0.465}$Li0.07) NbO₃–$x$SrZrO₃ ($x=0$, 0.005, 0.050, 0.095 and 0.14) were synthesized by the conventional solid-state reaction technique. XRD, SEM, densification parameter as well as dielectric parameters of the obtained KNLN ceramics were investigated. XRD analysis showed the formation of orthorhombic perovskite structure in all investigated specimens. SrZrO₃ (SZ) inhibited the grain growth and increased the distribution of grains uniformly in the microstructure. In addition, SZ changed the crystalline system of KNLN ceramics abnormally. As a consequence, the permittivity, dielectric loss as well as AC conductivity decreased remarkably. The ceramics of $x=0.005$ are characterized by low conductivity (10${}^{-10}$S/cm), low dielectric loss (0.005) and relatively high permittivity (34). They can be useful for the electrical capacitors fabrication and filtering out noise from signals in resonant circuits. At frequencies lower than 10⁵Hz, DC conductivity and separation of the charges at the interfaces were found to dominate the electrical behavior of ceramics, whereas at higher frequencies, a faster or dipole polarization may become more dominated. A broad peak has been observed in the imaginary part of electric modulus revealing to the non-Debye dielectric relaxation behavior. Quantum mechanical tunneling (QMT) was found to be the most suitable to characterize the electrical conduction mechanism in the ceramic compounds.

Sintered lead-free ${\text{Bi}}_{0.5}$(${\text{Na}}_{0.8}$${\text{K}}_{0.2}{)}_{0.5}$(${\text{Ti}}_{0.96}$${\text{Sn}}_{0.04})$O₃ ceramics (BNKTS) have been fabricated via a solid-state reaction. The effect of sintering temperature on the structural, morphological, dielectric, ferroelectric and energy storage properties of BNKTS ceramics was investigated, and it was found that the electrical properties of the synthesized ceramics increased with the increase in the sintering temperature, and the highest values were achieved at $1100^{\circ }$C. The ceramics sintered at the optimized temperature of $1100^{\circ }$C exhibited the best physical, dielectric, ferroelectric and energy storage properties, namely, high density (the relative density, $\rho =5.88$g.cm${}^{-3}$, approximate to 96.7% of the theoretical value), high densification factor ($\mathrm{\text{DF}}=0.93$), high dielectric constant (${\epsilon }_r=1215$), low dielectric loss (tan$\delta =0.051$), highest dielectric constant (${\epsilon }_{max}=4335$), high remanent polarization ($P_r=9.5\mu$C.cm${}^{-2})$, high coercive field ($E_c=14.3$kV/cm), high energy storage density (0.12J/cm${}^3)$, and high energy storage efficiency (41.7% at 46.3kV/cm).

New lead-free piezoceramic nanocomposites of Boron Sodium Gadolinium Niobate (BNGN), with general formula (1$-x)$${\text{B}}_{0.5}$${\text{Na}}_{0.5}$${\text{GdO}}_3-$$x$${\text{B}}_{0.5}$${\text{Na}}_{0.5}$NbO₃, exhibiting a Morphotropic Phase Boundary (MPB), have been synthesized following hydrothermal method followed by solid state sintering. The occurrence of MPB at the composition with $x=0.55$, at which rhombohedral and monoclinic phases are found to coexist, has been confirmed using powder XRD. This accounts for the occurrence of large remnant polarization when the sintered ceramic pellets are subjected to electric poling at 2KV/mm. Uniform microstructure of various compositions is confirmed by SEM imaging. Dielectric and piezoelectric properties of the samples are found to be comparable to those of commercial grade PZT. At the MPB, the ${\text{d}}_{33}$ coefficient is found to be 556 pC/N, which is close to that of commercial grade PZT, which makes BNGN a promising material to substitute lead containing PZT in the near future.

BaTiO₃ nanoparticles were synthesized by hydrothermal method using amorphous phase TiO₂ precursor as the Ti-source. The microstructure and phase structure were determined using XRD, SEM and Raman spectroscopy analysis results. The results showed that BaTiO₃ nanoparticles have tetragonal structure, average size of about 100 nm was obtained at Ba/Ti ratio of 1.5, synthesis temperature of 200^∘C and reaction time of 12 h. The components of the BaTiO₃ lead-free ceramic system are fabricated by conventional solid-phase reaction from the average size BaTiO₃ particles about 100 nm obtained by hydrothermal process. The effects of sintering behavior on dielectric, ferroelectric and piezoelectric properties of BT high-density ceramic were studied. The BaTiO₃ ceramic composition sintered at 1300^∘C has a relative density of 97%, the value of the electromechanical coefficient $k_p$ = 0.40, $k_{33}$ = 0.42, the large piezoelectric coefficient $d_{33}$ = 300 pC/N, $d_{31}=-$125 pC/N.

Ferroelectric materials are considered to be the most competitive energy storage materials for applications in pulsed power electronics due to excellent charge–discharge properties. However, the low energy storage density is the primary problem limiting their practical application. In this study, (1$-x$)Na${}_{0.5}$Bi${}_{0.5}$TiO₃–$x$Sr${}_{0.7}$La${}_{0.2}$TiO₃[(1$-x$)NBT–$x$SLT] ferroelectric ceramics are found to exhibit excellent energy storage performances through a synergistic strategy. As the SLT concentration increases, the relaxation characteristic increases significantly and the breakdown strength increases dramatically from 150 kV/cm to 220 kV/cm. The recoverable energy storage density of the 0.55NBT–0.45SLT ceramic is 2.86 J/cm³ with an energy storage efficiency of 88% under an electric field of 220 kV/cm. Furthermore, the ceramic with $x$ = 0.45 mol exhibited excellent energy storage stability in the ranges of 20–180${}^{\circ }$C (temperature) and 1–125 Hz (frequency). These excellent properties demonstrate the potential of (1$-x$)NBT–$x$SLT ceramics when used as dielectric capacitors in pulsed power systems.

This paper studies the dielectric spectra of solid solutions (SS) of the system (1 − x − y)NaNbO₃–$x$KNbO₃ –$y$CdNb₂O₆$y$= 0.075, $x$= 0.05 ÷ 0.30 in the temperature range (10 ÷ 900) K. The formation of a local maximum was established in the interval (260 ÷ 300) K at small $x$ values, which, as KNbO₃ increases, is gradually blurred and becomes an inflection point. Detected in SS with $x$ = 0.05 ÷ 0.10, the shift of the maxima of dependences $𝜀$^′/ $𝜀$₀($T$) and $𝜀$^′′/$𝜀$₀ ($T$), depending on the frequency of the electric field at the temperature ranges (300 ÷ 304) K and (258 ÷ 271) K, is not related to relaxation. This anomaly may indicate a crystallographic disorder to A and B positions. The conclusion is made about the expediency of using the obtained results for the development of functional ferroactive materials.

The lead-free ceramics ($1-x$)(0.94${\text{Bi}}_{0.47}$${\text{Na}}_{0.47}$${\text{Ba}}_{0.06}$TiO₃-0.06BiAlO₃)-$x$AgNbO₃ (denoted as BNBTA-$x$AN) were synthesized via a solid-state sintering method. The effect of AgNbO₃ doping amount on dielectric properties of the ceramics was studied systematically. X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscope were used to detect the structure of the ceramics. Temperature-dependent dielectric spectra, frequency-dependent dielectric constant and alternating current (ac) electric conductance at various temperatures were measured. The doping of AgNbO₃ greatly reduces dielectric constant around Curie temperature and thus enhances the temperature stability of the dielectric constant. The ceramic BNBTA-0.03AN exhibits excellent temperature-stable dielectric properties with temperature coefficient of capacitance (TCC) $\le \pm$15% between 55^∘C and 418^∘C with temperature window 363^∘C and small changes of dielectric constant and dielectric loss from 100 Hz to 1 MHz at different temperatures. The obtained ceramics are expected to be used in high-temperature capacitors due to its excellent temperature stability.