A novel ((Bi0.5Na0.5)0.94Ba0.06)1-x (K0.5Nd0.5)xTiO3 lead-free relaxor ferroelectric ceramic with large electrostrains at wide temperature ranges

Novel ((Bi_0.5Na_0.5)_0.94Ba_0.06)_1-x(K_0.5Nd_0.5)_xTiO₃(x = 0.0, 0.02, 0.04, 0.06) lead-free ceramics (BNBT–xKN) were prepared by the solid-state reaction method. The effects of A-site (K_0.5Nd_0.5)²⁺ complex-ion substitution on their phase structure, dielectric, piezoelectric, and electromechanical properties were studied. The X-ray diffraction results indicate that all compositions are located in the morphotropic phase boundary (MPB) region where the tetragonal phase coexists with the rhombohedral phase. In addition, as the KN content increases, the ferroelectric order transform to relaxor order, which is characterized by a degeneration of maximum polarization, remnant polarization and correspondingly adjusts the ferroelectric-relaxor transformation temperature (T_F-R) to room temperature. Interestingly, the disruption of ferroelectric phase caused a significant improvement of strains. A maximum strain of ~ 0.52% corresponding to normalized strain of ~ 612 pm/V appeared at 85 kv/cm for the x = 0.04 composition. Particularly, the composition of x = 0.04 exhibited high electrostrains of temperature insensitivity, which remained above 0.4% and kept within 10% from ambient temperature up to 110 °C. It can be ascribed to the coexistence of non-ergodic and ergodic states in the relaxor region. As a result, the systematic investigations on the BNBT–xKN ceramics can benefit the developments of temperature-insensitive “on-off” actuators.     

Large strain response in Li/Nb co-doped Bi0.5(Na0.8K0.2)0.5TiO3 lead-free piezoceramics

A series of (Bi_0.5Na_0.4K_0.1)Ti_0.98Nb_0.02O₃-xLi lead-free ceramics were fabricated using the solid-state reaction technique. The effects of Li/Nb cations on the structural and electrical properties of the ceramics were investigated. All the sintered ceramics exhibited pure perovskite structure and the average grain size increased slightly with increasing the Li content. Shape of the P-E loops illustrated the relaxor characteristic of all the samples. A giant strain of 0.4% was obtained at 60 kV/cm at x = 0.01 and the corresponding normalized strain was up to 683 pm/V, moreover, the strain exhibits excellent fatigue-resistance behavior. The giant strain can be attributed to the ferroelectric-relaxor phase transition under external driving electric field. These results indicate the sintered Li/Nb co-doped lead-free ceramics can be promising candidate for actuator applications.     

Enhanced energy-storage properties of (1-x)(0.7Bi0.5Na0.5TiO3-0.3Bi0.2Sr0.7TiO3)-xNaNbO3 lead-free ceramics

A series of (1-x)(0.7Bi_0.5Na_0.5TiO₃-0.3Bi_0.2Sr_0.7TiO₃)-xNaNbO₃ (BNT-BST-100xNN) lead-free ceramics were fabricated using conventional solid-state reaction technique. The phase behavior, microstructure, dielectric, ac impedance and energy-storage properties of the sintered ceramics were systematically investigated. XRD patterns and surface SEM micrographs revealed the introduction of NaNbO₃ didn't change the perovskite structure of BNT-BST at low doping level. The NaNbO₃ doping gave rise to slimmer P-E loops and thus gained enhanced energy storage properties. Therefore, a maximum energy storage density of 1.03 J/cm³ was achieved at 85 kV/cm at x = 0.01 via increasing the dielectric breakdown strength (DBS). Temperature-dependent dielectric permittivity illustrated the enhanced relaxor characteristics, implying the long-rang ferroelectric order was further damaged due to the introduction of NaNbO₃. The results above indicate the sintered ternary ceramics can be a promising lead-free candidate for energy storage capacitors.     

The phase formation process of Bi0.5(Na0.8K0.2)0.5TiO3 thin films prepared using the sol-gel method

Lead-free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ (abbreviated as BNKT) thin films were grown on Pt(111)/Ti/SiO₂/Si substrates using a sol-gel/spin coating technique and were then annealed at different temperatures (350 °C, 550 °C, 750 °C and 850 °C). Analysis of the XRD patterns and FT-IR spectra were used to determine the main reactions and the phase formation process of BNKT thin films during the sol-gel process. The results show that the dielectric constant of the thin films attains a maximum at a set temperature and then decreases at higher annealing temperatures, which can be attributed to phase formation and transformation. Moreover, the morphologies of the BNKT thin films improve with the increase in grain size and the formation of distinct grain boundaries. Furthermore, through increasing the pH of the precursor solutions, the size of the sol-gel colloidal particles increases slightly and the grains formed from the corresponding solutions tend to be small and uniform.     

Sol-gel processing and nanoscale characterization of (Bi0.5Na0.5)TiO3-SrTiO3 lead-free piezoelectric thin films

(1-x)(Bi_0.5Na_0.5)TiO₃-xSrTiO₃ (BNT-xST) (0 ≤ x ≤ 0.4) thin films were fabricated using a sol-gel technique on Pt(111)/Ti/SiO₂/Si(100) substrates, which were investigated by piezoresponse force microscopy (PFM) and Raman spectroscopy. The composition-induced phase transition was analyzed by acquiring structural variations and the domain distribution on a local scale. Raman spectra showed phonon anomalies with peak broadening and shifting when increasing SrTiO₃ (ST) concentrations were used. Changes in the domain morphology with changes in the composition were observed, and grains smaller than 0.5 µm were observed at lower concentrations of x = 0–0.25, while larger grains appeared with increasing ST contents. The switching spectroscopy PFM (SS-PFM) results supported a ferroelectric (FE) to relaxor ferroelectric (RFE) phase transition at approximately x ≈ 0.3 by means of analyzing the parameters as a function of the composition including the piezoresponse parameters of hysteresis loops (D_max, D_rem) and amplitude butterfly loops (S_total, S_neg). Hence, these results demonstrated that the composition-driven FE to RFE phase transition behavior, which is consistent with the localized response behavior, is dependent on the ST content in bulk BNT-xST ceramics.     

Phase structure,piezoelectric, ferroelectric,and electric-field-induced strain properties of Nb-modified 0.8Bi0.5Na0.5TiO3−0.2Sr0.85Bi0.1TiO3 ceramics

0.8Bi_0.5Na_0.5Ti_(1-x)Nb_xO₃−0.2Sr_0.85Bi_0.1TiO₃ (BNT-SBT-xNb, x = 0.00, 0.01, 0.02, and 0.03) piezoelectric ceramics were prepared by traditional solid state reaction and the influence of Nb substitution on the phase structure, ferroelectric, piezoelectric, and electric-field-induced strain properties in BNT-SBT ceramics were studied. XRD results exhibited that Nb⁵⁺ ions could fully diffuse into BNT-SBT structure to form a solid solution when x = 0.01. P-E loops and S-E curves suggested that the ferroelectric phase transformed to ergodic relaxor state (FE-to-ER) with the increasing the amount of Nb additive, indicating the ferroelectric long-ranged order was disturbed by the excess of Nb. With increasing Nb doping, phase transition temperature from normal ferroelectric to ergodic relaxor (short for T_F-R) could be reduced from 120 °C to 40 °C. Furthermore, for sample with x = 0.01, the normalized strain d₃₃^* got a maximum value ~571 pm/V due to the phase transition from ergodic relaxor to ferroelectric (ER-to-FE) under electric field.     

Electric-field-temperature phase diagram and electromechanical properties in lead-free (Na0.5Bi0.5)TiO3-based incipient piezoelectric ceramics

In this work, the (1-x)(0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃)-xSrTiO₃ (NKBT-xST) incipient piezoelectric ceramics with x = 0–0.07 (0ST-7ST) were prepared by the solid-state reaction method and their structural transformation and electromechanical properties were investigated as a function of ST content. As the ST content increases, the long-range ferroelectric order is disrupted, and the ferroelectric-relaxor phase transition temperature (T_FR) shifts to around room temperature for NKBT-5ST ceramics, accompanied by a relatively high electrostrain of 0.3% at 6 kV/mm. The large strain response associated with the vanished ferroelectric properties around T_FR can be attributed to the reversible relaxor-ferroelectric phase transition. The electric-field-temperature (E-T) phase diagrams were established, and the transition between the two field-induced long-range ferroelectric states were found to take place via a two-step switching process through an intermediate relaxor state. The threshold electric field to trigger the conversion between ferroelectric state and relaxor state depends strongly on the dynamics of polarization relaxation, which is influenced by temperature and composition.     

Synthesis and electrical properties of lead-free piezoelectric Bi0.5Na0.5TiO3 thin films prepared by Sol-Gel method

Lead-free Bi_0.5Na_0.5TiO₃ (BNT) piezoelectric thin films were deposited on Pt/TiO_x/SiO₂/Si substrates by Sol-Gel method. A dense and well crystallized thin film with a perovskite phase was obtained by annealing the film at 700 °C in a rapid thermal processing system. The relative dielectric constant and loss tangent at 12 kHz, of BNT thin film with 350 nm thickness, were 425 and 0.07, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 9 μC/cm² and a coercive field of 90 kV/cm. Piezoelectric measurements at the macroscopic level were also performed: a piezoelectric coefficient (d_33effmax) of 47 pm/V at E = 190 kV/cm was obtained. The piezoresponse force microscopy data confirmed that BNT thin films present ferroelectric and piezoelectric behavior at the nanoscale level.     

The electrical properties of (1−x)(Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3)–xCaZrO3 lead-free piezoelectric ceramics

Lead-free (1−x)(0.0852Bi_0.5Na_0.5TiO₃–0.12Bi_0.5K_0.5TiO₃–0.028BaTiO₃)–xCaZrO₃ piezoelectric ceramics (BNT−BKT−BT−xCZ, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) were prepared by using a conventional solid-state reaction method. The effects of CZ-doping on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT−BKT−BT−xCZ system were systematically investigated. The polarization and strain behaviors indicated that the long-range ferroelectric order in the unmodified BNT−BKT−BT ceramics was disrupted by the increase of CZ-doping content, and correspondingly the depolarization temperature (T_d) shifted down from 109 °C to below room temperature. When x>0.03, accompanied with the drastic decrease in the remnant polarization (P_r) and piezoelectric coefficient (d₃₃), the electric-field-induced strain was enhanced significantly. A large unipolar strain of 0.35% under an applied electric field of 70 kV/cm (S_max/E_max=500 pm/V) was obtained in the BNT−BKT−BT−0.04CZ ceramics at room temperature, which was attributed to the reversible electric-field-induced phase transition between the relaxor and ferroelectric phases.     

Enhanced dielectric and energy-storage properties in ZnO-doped 0.9(0.94Na0.5Bi0.5TiO3−0.06BaTiO3)−0.1NaNbO3 ceramics

[0.9(0.94Na_0.5Bi_0.5TiO₃?0.06BaTiO₃)?0.1NaNbO₃]-xZnO (NBT-BT-NN-xZnO, x=0, 0.5 wt%, 1.0 wt%, 1.5 wt%, and 2.0 wt%) ferroelectric ceramics were fabricated using a conventional solid-state reaction method. The effects of ZnO content on dielectric, energy-storage and discharge properties were systematically investigated. Dielectric constant and difference between maximum and remanent polarization were significantly improved by ZnO doping. Dielectric constant of NBT-BT-NN-1.0-wt% ZnO was 3218 at 1 kHz and room temperature, i.e. one time bigger than that of pure NBT-BT-NN ceramic. As a consequence, a maximum energy-storage density of 1.27 J/cm³ with a corresponding efficiency of 67% was obtained in NBT-BT-NN-1.0-wt% ZnO ceramic. Moreover, its pulsed discharge energy density was 1.17 J/cm³, and 90% of which could be released in less than 300 ns. Therefore, ZnO doped NBT-BT-NN ceramic with a large energy-storage density and short release time could be a potential candidate for applications in high energy-storage capacitors.     

Tuning the ferroelectric-relaxor transition temperature in NBT-based lead-free ceramics by Bi nonstoichiometry

In this study, the Bi-nonstoichiometric 0.99Bi_x(Na_0.8K_0.2)_0.5TiO₃-0.01SrTiO₃ (BNKST) ceramics with x = 0.5–0.535 mol (Bi50-Bi53.5) were prepared by a conventional solid-state reaction method. The effects of Bi excess on structural transition and ferroelectric stability of BNKST ceramics were systematically investigated by the Raman spectra, dielectric analyses and electromechanical measurements. The introduction of excess Bi³⁺ could significantly break the long-range ferroelectric order and favor the presence of relaxor phase, then the ferroelectric-relaxor transition temperature (T_FR) can be effectively tuned to around room temperature by Bi nonstoichiometry, giving rise to an enhanced room-temperature strain property. The positive strain S_pos and dynamic piezoelectric constant d₃₃^* of Bi52.5 critical composition reach 0.33% and 440 pm/V, respectively at 6 kV/mm. The high recoverable strain of Bi52.5 sample can be attributed to the electric-field-induced reversible relaxor-ferroelectric phase transition. The present work may be helpful for further understanding and designing high-performance NBT-based lead-free ceramics for piezoelectric actuator applications.     

Phase structure,ferroelectric properties,and electric field-induced large strain in lead-free 0.99[(1−x)(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5)TiO3]–0.01Ta piezoelectric ceramics

Lead-free 0.99[(1−x)(Bi_0.5Na_0.5)TiO₃-x(Bi_0.5K_0.5)TiO₃]–0.01Ta piezoelectric ceramics were prepared by a conventional solid-state reaction process. The ferroelectric properties, and strain behaviors were characterized. Increase of the (Bi_0.5K_0.5)TiO₃ content induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic phase. Accordingly, the ferroelectric order is disrupted significantly with the increase of (Bi_0.5K_0.5)TiO₃ content and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of 0.35% (corresponding to a large signal d₃₃ of 438 pm/V) in samples with 20 mol% (Bi_0.5K_0.5)TiO₃ content. Temperature dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced nonpolar pseudocubic-to-polar ferroelectric phase transformation.     

Tetragonal Er3+-doped (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)O3: Lead-free ferroelectric transparent ceramics with electrical and optical multifunctional performances

The lead-free Er³⁺-doped (K_0.48Na_0.48Li_0.04)(Nb_0.96Bi_0.04)O₃ (KNLNB-Er-x) ceramics were fabricated by conventional pressureless sintering. They possess a tetragonal perovskite phase with dense microstructure. High transmittances of the ceramics are obtained both in the visible and infrared regions. The optical band gap energies of them are 3.07–3.11 eV, close to other KNN-based materials. The relaxor-like characteristics, good dielectric, ferroelectric and piezoelectric properties of the ceramics have been obtained. The up-conversion photoluminescence spectra have been studied and obvious color-tunable emissions have been observed by modulating temperature. The fluorescence intensity ratio (FIR) value based on green emissions at 533 and 555 nm in the temperature ranging from 300 to 750 K have been investigated, giving the maximum sensitivity of ~ 0.0038 K^?1. Furthermore, the FIR technique opens up a method to detect the Curie temperature of the ceramics. Owing to both optical and electrical properties, the KNLNB-Er-x transparent ceramics could be a promising candidate in the application of color-tunable solid-state lightings, optical temperature sensors, and electrical-optical coupling devices.     

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.     

Low-temperature-sintered Bi0.5Na0.5TiO3-based lead-free ferroelectric ceramics with good piezoelectric properties

Li₂CO₃ has been used as a sintering aid for fabricating lead-free ferroelectric ceramic 0.93(Bi_0.5Na_0.5TiO₃)-0.07BaTiO₃. A small amount (0.5 wt%) of it can effectively lower the sintering temperature of the ceramic from 1200 °C to 980 °C. Unlike other low temperature-sintered ferroelectric ceramics, the ceramic retains its good dielectric and piezoelectric properties, giving a high dielectric constant (1570), low dielectric loss (4.8%) and large piezoelectric coefficient (180 pC/N). The “depolarization” temperature is also increased to 100 °C and the thermal stability of piezoelectricity is improved. Our results reveal that oxygen vacancies generated from the diffusion of the sintering aid into the lattices are crucial for realizing the low temperature sintering. Owing to the low sintering temperature and good dielectric and piezoelectric properties, the ceramics, especially of multilayered structure, should have great potential for practical applications.     

Effects of TiO2 addition on dielectric and energy storage properties of BaO-K2O-Nb2O5-SiO2 glass ceramics

In this work, 25.6BaO-6.4K₂O-32Nb₂O₅-36SiO₂-xTiO₂ (0 ≤ x ≤10 mol%) (BKNST) glass ceramics were synthesized by conventional melts and controllable crystallization method. The effects of different TiO₂ addition on the phase composition, dielectric and energy storage properties of BKNS glass ceramics were systematically evaluated. With the TiO₂ concentration increasing, a growing content of Ba₂TiO₄ phase was observed in the glass ceramics. The microstructures appeared to be homogenous and uniform with very low porosity through the addition of TiO₂, for which the maximal breakdown strength of 2112 kV/cm and the corresponding energy storage density of 9.48 J/cm³ were obtained with x = 7.5. The extremely low dielectric loss of less than 1‰ (25 °C, 100 kHz) and the obviously improved microstructure contributed to the increased breakdown strength. In addition, the discharge power density of the glass-ceramic capacitor (x = 7.5) was investigated using the RLC charge-discharge circuit and a relatively high value of 16 MW/cm³ at 300 kV/cm was obtained.     

High performance lead-free piezoelectric 0.96(K0.48Na0.52)NbO3-0.03[Bi0.5(Na0.7K0.2Li0.1)0.5]ZrO3-0.01(Bi0.5Na0.5)TiO3 single crystals grown by solid state crystal growth and their phase relations

0.96(K_0.48Na_0.52)NbO₃-0.03[Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5]ZrO₃-0.01(Bi_0.5Na_0.5)TiO₃ single crystals were grown for the first time by the solid state crystal growth method, using [001] or [110]-oriented KTaO₃ seed crystals. The grown single crystal shows a dielectric constant of 2720 and polarization-electric field loops of a lossy normal ferroelectric, with P_r = 45 μC/cm² and E_c = 14.9 kV/cm, while the polycrystalline samples with a dielectric constant of 828 were too leaky for P-E measurement due to humidity effects. The single crystal has orthorhombic symmetry at room temperature. Dielectric permittivity peaks at 26 °C and 311 °C, respectively, are attributed to rhombohedral-orthorhombic and tetragonal–cubic phase transitions. Additionally, Raman scattering shows the presence of an orthorhombic-tetragonal phase transition at ∼150 °C, which is not indicated in the permittivity curves but by the loss tangent anomalies. A transition around 700 °C in the high temperature dc conductivity is suggested to be a ferroelastic-paraelastic transition.     

Lead-free Bi1/2(Na0.82K0.18)1/2TiO3 relaxor ferroelectrics with temperature insensitive electrostrictive coefficient

The electric field-induced strain of Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ (BNKT) ceramics modified with BaZrO₃ (BZ) was investigated as a function of composition and temperature. Unmodified BNKT ceramics revealed a typical ferroelectric butterfly-shaped bipolar S–E loop at room temperature, whose normalized strain (S_max/E_max) showed a significant temperature coefficient of 0.38 pm/V/K. As the BZ content increased in the solid solution up to 5 mol%, the ferroelectric BNKT gradually transformed to a relaxor. Finally, 5 mol% BZ-modified BNKT ceramics showed a typical electrostrictive behavior with a thermally stable electrostrictive coefficient (Q₃₃) of 0.025 m⁴/C², which is comparable to that of Pb(Mg_1/3Nb_2/3)O₃ (PMN) ceramics that have been primarily used as Pb-based electrostrictive materials.     

Structure–property relationships of perovskite-structured Ca0.61Nd0.26Ti1-x(Cr0.5Nb0.5)xO3 ceramics

A series of Ca_0.61Nd_0.26Ti_1-x(Cr_0.5Nb_0.5)_xO₃ (CNTCNx) (0 ≤ x ≤ 0.1) ceramics were prepared via a solid state reaction method. All CNTCNx samples were crystallized into the orthorhombic perovskite structure. The SEM micrographs indicated that the average grain sizes of samples depended on (Cr_0.5Nb_0.5)⁴⁺ concentration. And as (Cr_0.5Nb_0.5)⁴⁺ concentration increased, the average grain size of samples decreased significantly. The short range order (SRO) structure and structural distortion of oxygen octahedra proved to exist in CNTCNx crystals from Raman spectra analysis results. The microwave dielectric properties highly depended on the B-site bond strength, oxygen octahedra distortion, reduction of Ti⁴⁺ to Ti³⁺ and internal strain η. At last, the CNTCN0.06 ceramic sintered at 1400 °C for 4 h exhibited good and stable comprehensive microwave dielectric properties of ε_r = 92.3, Q × f = 13,889 GHz, τ_f = + 152.8 ppm/°C.     

Co2O3 substitution effects on the structure and microwave dielectric properties of low-firing (Zn0.9Mg0.1)TiO3 ceramics

Low-firing (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ (x = 0.02–0.10) (ZMC_xT) microwave dielectric ceramics with high temperature stability were synthesized via conventional solid-state reaction. The influences of Co₂O₃ substitution on the phase composition, microstructure and microwave dielectric properties of ZMC_xT ceramics were discussed. Rietveld refinement results show the coexistence of ZnTiO₃ and ZnB₂O₄ phases at x = 0.02–0.10. (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ ceramic with x = 0.06 (ZMC_0.06T) obtains the best combination microwave dielectric properties of: ε_r = 21.58, Q × f = 53,948 GHz, τ_f = ? 54.38 ppm/°C. For expanding its application in LTCC field, 3 wt% ZnO-B₂O₃-SiO₂ (ZBS) and 9 wt% TiO₂ was added into ZMC_0.06T ceramic, great microwave dielectric properties were achieved at 900 °C for 4 h: ε_r = 26.03, Q × f = 34,830 GHz, τ_f = ? 4 ppm/°C, making the composite ceramic a promising candidate for LTCC industry.