Dielectric,ferroelectric and field-induced strain response of lead-free BaZrO3-modified Bi0.5Na0.5TiO3 ceramics

Lead-free piezoelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xBaZrO₃ (BNT–BZ100x, with x = 0–0.10) were prepared using a conventional solid-state reaction method. The crystal structure, microstructure, dielectric, ferroelectric, and piezoelectric properties of BNT–BZ100x ceramics were studied as functions of different BZ content. X-ray diffraction patterns revealed that the BZ completely diffused in the BNT lattice in the studied composition range. An appropriate amount of BZ addition improved the dielectric, ferroelectric, and piezoelectric properties of BNT ceramics. The remanent polarization (P_r) and piezoelectric constant (d₃₃) increased from 22 μC/cm² and 60 pC/N for pure BNT to 30 μC/cm² and 112 pC/N for x = 0.040, respectively. In addition, electric field-induced strain was enhanced to its maximum value (S_max = 0.40%) with normalized strain (d*₃₃ = S_max/E_max = 500 pm/V) at an applied electric field of 8 kV/mm for x = 0.055. The enhanced strain can be attributed to the coexistence of ferroelectric and relaxor ferroelectric phases.     

Synthesis and ferroelectric behavior of Gd doped BNT ceramics

In the present work, polycrystalline (Bi_1−xGd_x)_0.5Na_0.5TiO₃ (BGNT) ceramics with low amount of rare earth ion Gd³⁺ (x = 0, 0.02, 0.03, 0.04) have been synthesized by a semi–wet technique. XRD patterns show single phase formation for all the samples with a rhombohedral structure at room temperature. FE-SEM images show decrease in grain size with Gd concentration. The temperature dependence of dielectric constant has revealed that the depolarization temperature ‘T_d’ decreases with increasing x and the temperature ‘T_m’ of maximum dielectric constant increases initially for x = 0.02 thereafter decreases. All the samples have shown saturated hysteresis (P–E) loop at room temperature. The BGNT ceramic system for composition, x = 0.02 exhibits improved piezoelectric properties and strong ferroelectricity. With increasing temperature, polarization has been found to be reduced and deformed P–E loops are observed around ‘T_d’.     

Synthesis and characterizations of BNT–BT–KNN ceramics for energy storage applications

Dielectric, ferroelectric and piezoelectric properties of the (0.94–x) Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–xK_0.5Na_0.5NbO₃/BNT–BT–KNN ceramics with x = 0.02 and 0.05 (2KNN and 5KNN) were studied in detail. Dielectric study and temperature-dependent polarization hysteresis loops indicated a ferroelectric-to-antiferroelectric transition at depolarization temperature (T_d). The low T_d in both the ceramic samples suggested the dominant antiferroelectric ordering at room temperature (RT), which was also confirmed by RT polarization and strain hysteresis loops studies. Antiferroelectric-to-paraelectric phase transition temperature (T_m) was nearly same for both systems. The 5KNN ceramic samples showed the relaxor behaviour. The values of the dielectric constant, T_d, and maximum strain percentage increased, whereas the coercive field and remnant polarization decreased with the increase of the KNN percentage in the BNT–BT–KNN system. High-energy storage density ～0.5 J/cm³ at RT hinted about the suitability of the 5KNN system for energy storage applications.     

Enhanced depolarization temperature in 0.90NBT–0.05KBT–0.05BT ceramics induced by BT nanowires

The depolarization temperature (T_d) of piezoelectric materials is an important figure of merit for their application at elevated temperatures. This study focuses on the effect of BaTiO₃ (BT) nanowires on T_d and piezoelectric properties of morphotropic-phase-boundary 0.90NBT–0.05KBT–0.05BT ceramics. The results reveal that BaTiO₃ nanowires can pin the domain wall, leading to the increase of coercive field (E_c) from 21.06 kV/cm to 34.99 kV/cm. The T_d value of 0.90NBT–0.05KBT–0.05BT ceramics can be enhanced approximately 20 °C when using BT nanowires instead of BT solution as the raw material. Meanwhile, at the same polarization conditions, the piezoelectric constant of the ceramic added BT nanowires (172 pC/N) is decreased but still remains a larger value compared with those of other lead-free ceramics. The results imply that the addition of BT nanowires into NBT–KBT is a very effective route to improve T_d.     

Enhanced magnetocaloric effect in Eu-doped La0.7Ca0.3MnO3 compounds

Orthorhombic La_0.7-xEu_xCa_0.3MnO₃ samples (x = 0.04–0.12) with apparent density of ρ = 3.9–4.1 g/cm³ prepared by solid-state reactions have been studied. The analysis of temperature-dependent magnetization for an applied field H = 500 Oe indicated a decrease of the Curie temperature (T_C) from about 225 K for x = 0.04 through 189 K for x = 0.08–146 K for x = 0.12. The magnetocaloric (MC) study upon analyzing M(H, T) data has revealed that the magnetic entropy change around T_C reaches the maximum (|ΔS_max|), which is dependent on both x and H. For an applied field interval of ΔH = 60 kOe, |ΔS_max| values are about 5.88, 4.93, and 4.71 J/kg⋅K for x = 0.04, 0.08, and 0.12, respectively. Though |ΔS_max| decreases with increasing x, relative cooling power (RCP) increases remarkably from 383 J/kg for x = 0.04 to about 428 J/kg for x = 0.08 and 0.12. This is related to the widening of the ferromagnetic-paramagnetic transition region when x increases. Particularly, if combining two compounds with x = 0.04 and 0.08 (or 0.12) as refrigerant blocks for MC applications, a cooling device can work in a large temperature range of 145–270 K, corresponding to RCP ≈ 640 J/kg for H = 60 kOe. M(H) analyses around T_C have proved x = 0.04 exhibiting the mixture of first- and second-order phase transitions while x = 0.08 and 0.12 exhibit a second-order nature. The obtained results show potential applications of Eu-doped La_0.7Ca_0.3MnO₃ materials for magnetic refrigeration below room temperature.     

Enhanced piezoelectric properties near the morphotropic phase boundary in lead-free (1-x)(Bi0.5K0.5)TiO3-xBi(Ni0.5Ti0.5)O3 ceramics

Lead-free piezoelectric ceramics of the composition (1-x)(Bi_0.5K_0.50)TiO₃-xBi(Ni_0.50Ti_0.50)O₃ or (1-x)BKT-xBNiT (when x = 0–0.20 mol fraction) were prepared by a conventional mixed-oxide method and sintered at 1050 °C for 4 h. The effects of BNiT content on the phase equilibria, and the dielectric, ferroelectric and piezoelectric properties were systematically investigated. High density sintered specimens (5.71–6.12 g/cm³) were obtained for all compositions. X-ray diffraction patterns showed that all BKT-BNiT samples exhibited a single perovskite phase which confirms that BNiT and BKT formed a solid solution up to x = 0.20. A morphotropic phase boundary (MPB) separating a BKT-rich tetragonal phase and a BNiT pseudo-cubic phase was identified over the compositional range 0.05 < x < 0.10, where enhanced electrical properties were observed. The optimum dielectric properties (ε_r = 1710, tanδ = 0.036), ferroelectric properties (P_r = 16.6 μC/cm², E_c = 22.5 kV/cm and R_sq = 0.86) and piezoelectric properties (d₃₃ = 288 pC/N, S_max = 0.22% and d^*₃₃ = 313 pm/V) were observed with a relatively high T_m ∼ 304 °C within this MPB region. Overall, these results indicate that the BKT-BNiT ceramic system is a promising lead-free piezoelectric candidate for further development for actuator applications.     

Ferroelectric properties of Pb(Zr1/2Ti1/2)O3–Pb(Zn1/3Nb2/3)O3 ceramics under compressive stress

Effects of compressive stress on the ferroelectric properties of ceramics in PZT–PZN system were investigated. The ceramics with a formula (1−x)Pb(Zr_1/2Ti_1/2)O₃–xPb(Zn_1/3Nb_2/3)O₃ or (1−x)PZT–(x)PZN (x = 0.1–0.5) were prepared by a conventional mixed-oxide method. The ferroelectric properties under the compressive stress of the PZT–PZN ceramics were observed at the stress levels up to 170 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. It was found that with increasing compressive stress the area of the ferroelectric hysteresis (P–E) loops, the saturation polarization (P_sat), the remnant polarization (P_r), and the coercive field (E_c) decreased. These results were interpreted through the non-180° ferroelastic domain switching processes.     

Dielectric properties of PIN–PT ceramics under compressive stress

Lead indium niobate, Pb(In_1/2Nb_1/2)O₃ or (PIN), is an interesting ferroelectric material, because it can be changed from a disordered state to ordered state by long-time thermal annealing. However, the temperature related to the maximum dielectric constant (T_max) of PIN in relaxor phase is low (at 1 kHz, T_max = 66 °C). In this study, to increasing T_max of PIN, lead titanate, PbTiO₃(PT) was thus added to PIN with compositions (1 − x)PIN–xPT (for x = 0.1–0.5). The influence of stress on the dielectric properties of (1 − x)PIN–xPT ceramics was then investigated. The dielectric properties were measured under various uniaxial compressive stress up to 400 MPa. The results showed that the superimposed compression load reduced the dielectric constant in 0.9PIN–0.1PT. For the other compositions, the dielectric constants first increased with the compressive stress, and then decreased when the stress was further increased up to 400 MPa. The dielectric loss tangent of all composition was found to decrease with increasing compressive stress.     

Large electrostrictive strain in lead-free Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript>–KNbO<Subscript>3</Subscript> ceramics

In the present work, (1−x)(0.935Bi_0.5Na_0.5TiO₃–0.065BaTiO₃)–xKNbO₃ (BNT–BT–KN, BNT–BT–100xKN) ceramics with x ranging from 0 to 0.1 were prepared by the conventional ceramic fabrication process. A large electrostrictive coefficient of ∼10⁻² m⁴ C⁻² is obtained with the composition x ranging from 0.02 to 0.1, which is close to the well-known electrostrictive material Pb(Mg_1/3Nb_2/3)O₃. Under an electric field of 4 kV/mm, the electrostrictive strain can reach as high as 0.08%. Besides, the electric field induced strain behavior indicates a temperature independent behavior within the temperature range of 20 to 150°C. The large electrostrictive strain is suggested to be ascribed to the formation of non-polar (NP) phase developed by the KNbO₃ substitution, and the high electrostrictive coefficient of BNT–BT–KN ceramics makes them great candidates to be applied in the new solid-state actuators.     

Dielectric and electromechanical properties of (1−x)PMN-xPZT ferroelectric ceramics

The dielectric and electromechanical characteristics of the (1?x)PMN-xPZT ferroelectric ceramics have been obtained at different temperatures, amplitudes, and frequencies of the measuring field and at different bias field strengths. It is shown that this ferroelectric ceramics at low and infralow frequencies possesses pronounced relaxor properties in a certain temperature range and ferroelectric properties in other temperature range. The temperature and amplitude ranges have been determined, in which the permittivity ?′ either only decreases or first increases and then decreases with an increase in the measuring field amplitude E ₀. The temperature ranges of existing the phases similar to the superparaelectric phase, dipole glass phase, and ferroelectric phase are evaluated from the temperature dependences of the coercive field E _c (T) and the remanent polarization P _r (T) and also from the reverse dependences of ?* and the electromechanical characteristics. The PZT concentration in the PMN-PZT system is determined, at which the electrostrictive constant M ₁₁ is maximum. It is demonstrated that, in the neighborhood of the temperature at a maximum of ?′, the strain S ₃ is quadratic in the field E ₌; that is, S ₃=M ₁₁ E ².     

Dielectric and magnetoelectric properties of BaTiO3–CoMn0.2Fe1.8O4 particulate (0-3) multiferroic composites

Multiferroic properties of (x) CoMn_0.2Fe_1.8O₄–(1-x) BaTiO₃ particulate magnetoelectric (ME) composites with x = 0.1, 0.2, 0.3 M percentage was investigated. The CoMn_0.2Fe_1.8O₄ (CMFO) phase was synthesized by solution combustion route and BaTiO₃ (BT) phase was synthesized by wet chemical method. X-ray diffraction studies revealed the purity of constitute phases; confirmed the manifestation of CMFO and BT within the ME composite structure. The microstructural aspects were observed by using Fe-SEM; revealed the effect of constituent phases on the average grain size of the composites. The temperature dependent dielectric properties for BT exhibited the three anomalies associated to its crystallographic lattice structure change with temperature. Dielectric constant of the composite was found to be decreased with CMFO content. All the composite structures exhibited typical magnetic hysteresis nature at room temperature and showed linear effect on the saturation magnetization of the composite with CMFO content. The ME response was examined at room temperature with an ac magnetic field at 1 kHz, all the composite showed a sharp decreasing behavior of the ME voltage coefficient (α_ME) to an applied dc bias in low field region. The maximum α_ME factor of ～8.51 mV/cm Oe was observed for 10% CMFO–90% BT composition.     

Thermal expansion in the Burns-phase of barium titanate stannate

The thermal expansion S_T has been measured in the system BaTi_1−xSn_xO₃, both for the pure compositions x=0 (BT) and x=1 (BS), and for solid solutions 0.025≤x≤0.2 (BTS). For all ceramics examined, a non-linear temperature dependence S_T(T) has been observed at elevated temperatures This is related to thermally generated impurities and, below the Burns-temperature T_d of BT and BTS, to the non-linear strain contribution of polar nanoregions. With increasing Sn-content x, a steep increase of the Burns-temperature is found in BTS for compositions x≥0.025.     

The effects of sintering temperatures on dielectric,ferroelectric and electric field-induced strain of lead-free Bi0.5(Na0.78K0.22)0.5TiO3 piezoelectric ceramics synthesized by the sol–gel technique

Lead-free Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (BNKT) piezoelectric ceramics were synthesized by the sol–gel technique. The effects of sintering temperatures on the crystal structure, microstructure, densification, dielectric, ferroelectric and electric field-induced strain behaviors of the BNKT ceramics were investigated. X-ray diffraction patterns exhibited a pure perovskite structure from 1075 to 1150 °C. A scanning electron microscopy study revealed an increase in grain size with increasing sintering temperature. The density of the ceramics sintered at 1150 °C reaches a maximum value of 5.55 g/cm³, which is 96% of the theoretical density. BNKT ceramics sintered at an optimum temperature of 1150 °C exhibited a high remnant polarization of 18.5 μC/cm², a high electric field-induced strain of 0.20% and dynamic piezoelectric coefficient d₃₃¹ = (S_max/E_max) of 247 pm/V.     

Low-temperature preparation and properties of ceramics with composition (1−x)CaTiO3−xPbF2−xLiF

Fluorinated ceramics with initial composition (1−x)CaTiO₃+xPbF₂+xLiF were sintered at 950 °C. The X-ray diffraction (XRD) patterns of the samples showed the formation of a novel solid solution in the initial composition range 0⩽x⩽0.125. SEM observations were performed on fractured ceramics and DSC analyses were carried out from room temperature up to 600 °C. Three second-order phase transitions were detected for all the samples. Capacitors were prepared from the pre-sintered ceramics then dielectric measurements were performed as a function of temperature in the frequency range 10²–4×10⁷ Hz. The ε′_r−T curves exhibit the profile of dielectrics for class I capacitors, however the values of tan δ are too high (tan δ⩾1%).     

Magnetocaloric effect in Y-doped La0.6Ca0.4MnO3 enhanced by Griffiths phase and re-entrance of first-order phase transition

It has been known that bulk La_0.6Ca_0.4MnO₃ is an intermediate material of the first- and second-order characters with the tricritical-point exponents, and the doping of a metal ion in it usually causes a continuous second-order transition. The present work reports the re-entrance of a discontinuous first-order transition in orthorhombic La_0.6-xY_xCa_0.4MnO₃ (x = 0.03–0.09) compounds. This enhances the magnetocaloric effect. For the field H = 30 kOe, the maximum magnetic-entropy change (|ΔS_max|) and relative cooling power (RCP) have been evaluated being about 5.45–6.3 J/kg·K and 130–185 J/kg, respectively. If combining these compounds as refrigerant blocks in a rotary ring model, a magnetic cooling device can operate at temperatures T = 85–280 K, with |ΔS_max| ≈ 5.5 J/kg⋅K and RCP ≈ 1073 J/kg. Aside from the re-entranced first-order phase transition, the magnetization and structural analyses have proved the enhanced magnetocaloric effect in La_0.6-xY_xCa_0.4MnO₃ related to a Griffiths singularity, and local Jahn-Teller distortions of the perovskite structure (since the Mn³⁺/Mn⁴⁺ ratio and orthorhombic structural phase are unchanged vs. x).     

Effects of Zr/Ti ratio on dielectric and ferroelectric properties of 0.8Pb(ZrxTi1−x)O3–0.2Pb(Co1/3Nb2/3)O3 ceramics

The influences of Zr/Ti ratio on electrical properties of the 0.8Pb(Zr_xTi_1−x)O₃–0.2Pb(Co_1/3Nb_2/3)O₃ ceramics prepared by a mixed-oxide method (with x = 0.46, 0.48, 0.50, 0.52, and 0.54) have been investigated in order to identify the morphotropic phase boundary composition in this system. With XRD analysis, the crystal structure of dense specimens appeared to change gradually from tetragonal to rhombohedral with increasing Zr content. The dielectric properties measurements showed a maximum dielectric constant at x = 0.50, while the transition temperature decreased with increasing Zr content in the system. Moreover, all ceramics showed diffused phase transition behaviors with a minimum diffusivity at x = 0.50. In addition, the Polarization–Electric field (P–E) hysteresis loops of the ceramic systems also changed significantly with the Zr content. Interestingly, the loop squareness parameter reached maximum around x = 0.50. Other ferroelectric hysteresis parameters showed noticeable change at x = 0.50. These results clearly showed the significance of Zr/Ti ratio in controlling the electrical properties of the PZT–PCN ceramic systems.     

Electrical conduction and thermoelectric properties of perovskite-type BaBi1−xSbxO3

To elucidate the thermoelectric properties at high temperatures, the electrical conductivity and Seebeck coefficient were measured at temperatures between 423 K and 973 K for perovskite-type ceramics of BaBi_1?xSb_xO₃ solid solutions with x=0.0–0.5. All the ceramics exhibit p-type semiconducting behaviors and electrical conduction is attributed to hopping of small polaronic holes localized on the pentavalent cations. Substitution of Bi with Sb causes the electrical conductivity σ and cell volume to decrease, but the Seebeck coefficient S to increase, suggesting that the Sb atoms are doped as Sb⁵⁺ and replace Bi⁵⁺, reducing 6s holes conduction from Bi⁵⁺(6s⁰) to Bi³⁺ (6s²). The thermoelectric power factor S ²σ has values of 6×10^?8–3×10^?5 W m^?1 K^?2 in the measured temperature range, and is maximized for an Sb-undoped BaBiO_3?δ, but decreases upon Sb doping due to the decreased σ values.     

Investigation on bulk Nd–Fe–Al amorphous/nano-crystalline alloy

Cylindrical ingots of bulk amorphous Nd₇₀Fe₂₀Al₁₀ with a diameter of 8 mm were prepared by a copper mold casting method. It was proved experimentally with X-ray diffraction, scanning electron microscopy and differential scanning calorimetry that the as-prepared alloy samples consisted mainly of the amorphous phase with a minute amount of nano-crystalline phase. The onset crystallization temperature (T_x) and the melting temperature (T_m) of the samples were 743 and 823 K, respectively, from DSC results. The temperature interval between T_x and T_m, ΔT=T_m−T_x, is 80 K and the resulting ratio of T_x/T_m is 0.90. Both a high T_x/T_m ratio and a small ΔT are considered the reasons for the good glass-forming ability. The Curie temperature (T_c) of these samples was 525 K from magneto-thermal gravimetric analysis. This measured value is higher than the highest T_c among binary Nd–Fe amorphous alloys. Annealing treatments were carried out for the as-cast samples to obtain dual-phase samples with different volume fractions of nano-crystalline phase. Magnetic measurement results indicated that the hard magnetic behavior is weakest for samples with 40% of nano-crystalline phase. The curve of the measured hysteresis loop area versus the volume fraction of nano-crystalline phase concaves upward, which agrees with what we predicated in our previous simulation results.     

Dielectric and piezoelectric properties of non-stoichiometric (K0.5Na0.5)(Nb0.9+xTa0.1)O3 ceramics

In the study, in order to develop the lead-free piezoelectric ceramics for actuator, transformer and other electronic-devices application, (K_0.5Na_0.5)(Nb_0.9+xTa_0.1)O₃ + 0.5 mol% CuO + 0.2 mol% MnO₂ ceramics were prepared by conventional mixed oxide method. The effects of B-site non-stoichiometry in [(K_0.5Na_0.5)] [(Nb_0.9+xTa_0.1)O₃] ceramics on microstructure and piezoelectric properties were investigated. The density, electromechanical coupling factor (k_p), mechanical quality factor (Q_m), piezoelectric constant (d₃₃), T_C and T_O-T of NKNT ceramics with x = 0.0065 showed the optimum values of 4.58 g/cm³, 0.427, 1554, 109 pC/N, 373 °C and 226 °C, respectively, suitable for piezoelectric motor, and transformer applications.     

Dielectric properties Ca-substituted barium strontium titanate ferroelectric ceramics

The dielectric and ferroelectric properties of Ba_1−xSr_xTiO₃ (BST) (x=0.10,0.20,0.30,0.40 and 0.60) ceramics and Ba_1−2xSr_xCa_xTiO₃ (BSCT) (x=0.10,0.20,0.30) ceramics have been investigated. The low temperature phase transitions of BST ceramics vanish after Ca²⁺ substitution while the high temperature transition is diffused and relaxed, which becomes more obvious with increasing x. Ca²⁺ substitution obviously decreases the dielectric constant maximum, K_m, of BST ceramics and changes the temperature of dielectric constant maximum, T_m, of BST ceramics. The shift of T_m in BST is attributed mainly to the Sr²⁺ and Ba²⁺ concentration. BST ceramics exhibit almost normal ferroelectric characteristics, while a typical relaxor behavior was observed in BSCT ceramics. The relaxor behavior observations may be understood by a random electric field induced domain state.