Tailoring Strain Properties of (0.94−x)Bi1/2Na1/2TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 Ferroelectric/Relaxor Composites

A remarkable progress in the quest of lead‐free piezoceramics for actuator applications has been made with the development of incipient piezoceramics featured by giant strains. A drawback, however, is the high electric field required to generate this giant strain. A powerful approach to overcoming this drawback lies in relaxor/ferroelectric (FE) composites comprised such giant strain materials (matrix) and a FE or nonergodic relaxor (seed). In this study, we investigate the effect of K_0.5Na_0.5NbO₃ content in the matrix and the volume ratio of seed to matrix using composites of 0.93Bi_1/2Na_1/2TiO₃–0.07BaTiO₃ as a seed and (0.94 ? x)Bi_1/2Na_1/2TiO₃–0.06BaTiO₃–xK_0.5Na_0.5NbO₃ as a matrix. The strain of all matrices, independent of their K_0.5Na_0.5NbO₃ content, was found to be enhanced by adding a certain amount of seed. An optimum strain is achieved for the composite comprised of a matrix with x = 0.02 K_0.5Na_0.5NbO₃ and 10% seed. By means of a differential analysis on the temperature‐dependent dielectric permittivity, it was shown that the seed phase is still present in the composites despite the naturally expected diffusion process during sintering.     

Crack‐tip toughness of lead‐free (1−x)(Na1/2Bi1/2)TiO3–xBaTiO3 piezoceramics

Crack opening displacements were evaluated on semi‐elliptical indentation cracks in lead‐free (1?x)(Na_1/2Bi_1/2)TiO₃–xBaTiO₃ piezoceramics and a commercially available PZT ceramic. The observed crack‐tip toughness of NBT‐xBT was found to be substantially higher than for PZT. Two evaluations for the crack opening displacement were demonstrated and contrasted: A more elaborate three‐term‐approximation and a pragmatic utilization of the Irwin parabola.     

Large strain response with low driving field in Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3–Bi(Mg2/3Nb1/3)O3 ceramics

The (1?x)(0.8Bi_1/2Na_1/2TiO₃–0.2Bi_1/2K_1/2TiO₃)?xBiMg_2/3Nb_1/3O₃ (100xBMN) ternary solid solutions were designed and prepared using a conventional solid‐state reaction. Temperature and compositional dependent ferroelectric, piezoelectric, dielectric features, and structural evolution were systematically studied. At the critical composition of 2BMN, a large bipolar strain of 0.43% was achieved at 55 kV/cm, and the normalized strain reaches to 862 pm/V at a low driving electric field of 40 kV/cm. It was found that the substitution of BiMg_2/3Nb_1/3O₃ induces a transformation from ferroelectric to relaxor phase by disrupting the long range ferroelectric order. Therefore, as the external electric field was applied, a relaxor‐ferroelectric phase transition will be induced. This is contributed to the giant strain. The results above suggest that such a ternary composition is a promising candidate for application to actuator.     

Composition‐ and Temperature‐Dependent Large Strain in (1−x)(0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3)– xNaNbO3 Ceramics

Ternary solid solutions of (1 ? x)(0.8Bi_0.5Na_0.5TiO₃–0.2Bi_0.5K_0.5TiO₃)– xNaNbO₃ (BNKT–xNN) lead‐free piezoceramics were fabricated using a conventional solid‐state reaction method. Pure BNKT composition exhibited an electric‐field‐induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature. Accompanied with the ferroelectric‐to‐relaxor temperature T_F‐R shifted down below room temperature as the substitution of NN, a compositionally induced nonergodic‐to‐ergodic relaxor transition was presented, which featured the pinched‐shape polarization and sprout‐shape strain hysteresis loops. A strain value of ~0.445% (under a driving field of 55 kV/cm) with large normalized strain of ~810 pm/V was obtained for the composition of BNKT–0.04NN, and the large strain was attributed to the reversible electric‐field‐induced transition between ergodic relaxor and ferroelectric phase.     

High permittivity (1−x)Bi1/2Na1/2TiO3‐xPbMg1/3Nb2/3O3 ceramics for high‐temperature‐stable capacitors

(1?x)Bi_1/2Na_1/2TiO₃‐xPbMg_1/3Nb_2/3O₃[(1?x)BNT‐xPMN] ceramics have been fabricated via a conventional solid‐state method for compositions x ≤ 0.3. The microstructure, phase structure, ferroelectric, and dielectric properties of ceramics were systematically studied as high‐temperature capacitor materials. XRD pattern certified perovskite phase with no secondary phase in all compositions. As PMN concentration increased, the phase of (1?x)BNT‐xPMN ceramics transformed from ferroelectric to relaxor gradually at room temperature, with prominent enhancement of dielectric temperature stability. For the composition x = 0.2, the temperature coefficient of capacitance (TCC) was <15% in a wide temperature range from 56 to 350°C with high relative permittivity (>3300) and low dielectric loss (<0.02) at 150°C, which indicated promising future for (1?x)BNT‐xPMN system as high‐temperature stable capacitor materials.     

Orientation‐Dependence of Thermal Depolarization and Phase Development in Bi1/2Na1/2TiO3‐BaTiO3 Single Crystals

The direction‐dependence of pyroelectric properties of (1 ? x)Bi_1/2Na_1/2TiO₃ ? xBaTiO₃ (BNT ? 100xBT) is investigated, using single crystal samples with well‐defined orientations for x = 0.036 and x = 0.063. The results are compared with those of temperature‐dependent measurements of the ferroelectric and dielectric hysteresis. The depolarization temperature T_d of each crystal composition is found to depend on crystal orientation, a fact that is explained by differences in the stability of respective domain configurations. A rationalization is offered for the observation that T_d differs from the ferroelectric‐relaxor transition temperature, depending on orientation. The hysteresis curves of BNT ? 3.6BT are typical for a rhombohedral system with a ferroelectric‐relaxor transition, with polarization reversal close to T_d occurring in a multistep process that includes decay of ferroelectric domains into polar nanoregions and re‐formation of domains. BNT ? 6.3BT, a composition in the region of the morphotropic phase boundary, shows the same feature, but additionally is characterized by a field‐induced transition between rhombohedral and tetragonal symmetry. This combination results in an effective piezoelectric coefficient of pm/V.     

Macroscopic and Nanoscopic Polarization Relaxation Kinetics in Lead‐Free Relaxors Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3–BiZn1/2Ti1/2O3

The stability of the field‐induced ferroelectric (FE) state was studied in relaxor lead‐free ceramics (1 ? y)[0.81Bi_1/2Na_1/2TiO₃–0.19Bi_1/2K_1/2TiO₃]–yBiZn_1/2Ti_1/2O₃ both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn_1/2Ti_1/2)O₃, which are in the non‐ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two‐step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn_1/2Ti_1/2)O₃ content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature.     

Temperature‐Dependent Phase Transitions in the Lead‐Free Piezoceramics (1 – x – y)(Bi1/2Na1/2)TiO3–xBaTiO3–y(K0.5Na0.5)NbO3 Observed by in situ Transmission Electron Microscopy and Dielectric Measurements

Lead‐free piezoelectric (1 – x – y)(Bi_1/2Na_1/2)TiO₃–xBaTiO₃–y(K_0.5Na_0.5)NbO₃ (BNT–BT–KNN) ceramics were examined in situ under increasing temperature in the transmission electron microscope. Changing superstructure reflections indicate a transition from rhombohedral to tetragonal to cubic phase with broad coexistence regions. The additional evolution of the microstructure in combination with dielectric measurements leads to a model of two relaxor‐type phase evolutions with temperature.     

Role of sodium deficiency on the relaxor properties of Bi1/2Na1/2TiO3-BaTiO3

The influence of A-site deficiency on the relaxor properties in the lead-free (1-x)(Bi_1/2Na_1/2)TiO₃-xBaTiO₃ solid solution system was studied by intentionally reducing Na content in reference to the stoichiometric compositions. We observed that for all compositions, the higher the level of Na deficiency was, the lower the transition temperature from a ferroelectric to relaxor state became. The compositions with intermediate BaTiO₃ contents (x?=?0.06, 0.09, 0.13, and 0.40) showed sprout-shaped strains and pinched polarization curves at room temperature, indicating a crossover from a non-ergodic relaxor to ergodic relaxor state above a certain level of Na deficiency. We demonstrate that the presence of recently proposed oxygen octahedral tilt disorder as a prerequisite for the relaxor features in this system is not necessarily true, but the presence of stress-field-induced defect dipoles matters. A transmission electron microscopy confirmed that micro domains break down into nano domains with increasing Na deficiency level.     

Large Strain Response in 0.99(Bi0.5Na0.4K0.1)TiO3–0.01(KxNa1−x)NbO3 Lead‐Free Ceramics Induced by the Change of K/Na Ratio in (KxNa1−x)NbO3

Influence of K/Na ratio in (K_xNa_1?x)NbO₃ on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi_0.5Na_0.4K_0.1)TiO₃–0.01(K_xNa_1?x)NbO₃ (BNKT–K_xNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase 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, giant strain of ~0.46% (corresponding to a large signal d₃₃* of ~575 pm/V) which is comparable to that of Pb‐based antiferroelectrics is obtained at a K/Na ratio of ~1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric–ergodic relaxor phase boundary in the BNKT–K_xNN system and the tolerance factor t of the end number (K_xNN). In situ high‐energy X‐ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field‐induced distortion from the pseudocubic structure.     

Electrical properties and temperature stability of SrTiO3-modified (Bi1/2Na1/2)TiO3-BaTiO3-(K1/2Na1/2)NbO3 piezoceramics

SrTiO₃-modified lead-free piezoelectric ceramics, (0.93-x)Bi_0.5Na_0.5TiO₃-xSrTiO₃-0.06BaTiO₃-0.01 K_0.5Na_0.5NbO₃ [(BNT-xST)-BT-KNN, x = 0-0.06], were prepared using a conventional solid-state reaction method. The XRD structure analysis and electric properties characteristics revealed the ST-induced phase transformation from the ferroelectric phase to the relaxor phase and their coexistence state. Benefiting from the ST-destructed ferroelectric long-range orders, the high normalized strain value of 600 pm/V was obtained in the (BNT-0.02ST)-BT-KNN ceramic at 5 kV/mm. The ST-generated relaxor phase was found to have a constructive effect on improving the temperature stability and restraining the hysteresis of the electric-field-induced strain. The normalized strain of (BNT-0.06ST)-BT-KNN ceramics could be kept at a high value ~337 pm/V at elevated temperature up to 120°C.     

Tailoring the Piezoelectric and Relaxor Properties of (Bi1/2Na1/2)TiO3–BaTiO3 via Zirconium Doping

This article details the influence of zirconium doping on the piezoelectric properties and relaxor characteristics of 94(Bi_1/2Na_1/2)TiO₃–6Ba(Zr_xTi_1?x)O₃ (BNT–6BZT) bulk ceramics. Neutron diffraction measurements of BNT–6BZT doped with 0%–15% Zr revealed an electric‐field‐induced transition of the average crystal structure from pseudo‐cubic to rhombohedral/tetragonal symmetries across the entire compositional range. The addition of Zr up to 10% stabilizes this transition, resulting in saturated polarization hysteresis loops with a maximum polarization of 40 μC/cm² at 5.5 kV/mm, while corresponding strain hysteresis measurements yield a maximum strain of 0.3%. With further Zr addition, the ferroelectric order is progressively destabilized and typical relaxor characteristics such as double peaks in the current density loops are observed. In the strain hysteresis, this destabilization leads to an increase of the maximum strain by 0.05%. These changes to the physical behavior caused by Zr addition are consistent with a reduction of the transition temperature T_F‐R, above which the field‐induced transformation from the relaxor to ferroelectric state becomes reversible.     

Temperature‐insensitive strain behavior in 0.99[(1−x)Bi0.5(Na0.80K0.20)0.5TiO3−xBiFeO3]–0.01Ta lead‐free piezoelectric ceramics

Lead‐free 0.99[(1?x)Bi_0.5(Na_0.80K_0.20)_0.5TiO₃?xBiFeO₃]–0.01Ta (BNKT20–100xBF–1Ta) lead‐free piezoelectric ceramics were fabricated through conventional solid state sintering method. Results showed that change of BF content in the BNKT20–100xBF–1Ta induced a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long‐range ferroelectric order. A large electric‐field‐induced strain of 0.36% (at 80 kV/cm driving field, corresponding to a large signal of ~450 pm/V) which is derived from a reversible field‐induced ergodic relaxor to ferroelectric phase transformation, was obtained in the composition with x=0.01 near the ferroelectric‐ergodic relaxor phase boundary. Moreover, an attractive property for application in nonlinear actuators demanding enhanced thermal stability was obtained in this material, which showed a temperature‐insensitive strain characteristic in the temperature range from room temperature to 100°C.     

Electric field-induced changes in the ferroelastic behavior of (Na1/2Bi1/2)TiO3-BaTiO3

In this study, the macroscopic mechanical behavior was characterized for poled and unpoled polycrystalline (1?x)(Na_1/2Bi_1/2)TiO₃-xBaTiO₃ (NBT-xBT) for compositions across the morphotropic phase boundary (MPB). Due to a field-induced ferroelectric phase transformation, NBT-xBT compositions near the MPB (x?=?6–7?mol%) showed a significant decrease in the coercive stress for electrically poled samples. The apparent difference in mechanical behavior is suggested to be due to an irreversible electric-field-induced transformation to long-range ferroelectric order in the poled samples. The results indicate a significant difference in the critical stresses for the relaxor-ferroelectric transition and ferroelastic domain wall motion, which can have important effects on applications for lead-free ferroelectrics. To further illustrate this, a method was developed to electrically depole NBT-xBT at room temperature, resulting in an unpoled NBT-xBT material with long-range ferroelectric order. Mechanical testing revealed analogous macroscopic ferroelastic behavior to the poled samples, despite the lack of a piezoelectric response.     

Phase‐Composition‐Dependent Piezoelectric and Electromechanical Strain Properties in (Bi1/2Na1/2)TiO3–Ba(Ni1/2Nb1/2)O3 Lead‐Free Ceramics

New lead‐free perovskite solid solution ceramics of (1 ? x)(Bi_1/2Na_1/2)TiO₃–xBa(Ni_1/2Nb_1/2)O₃[(1?x)BNT–xBNN,x = 0.02–0.06) were prepared and their dielectric, ferroelectric, piezoelectric, and electromechanical properties were investigated as a function of the BNN content. The X‐ray diffraction results indicated that the addition of BNN has induced a morphotropic phase transformation from rhombohedral to pseudocubic symmetry approximately at x = 0.045, accompanying an evolution of dielectric relaxor behavior as characterized by enhanced dielectric diffuseness and frequency dispersion. In the proximity of the ferroelectric rhombohedral and pseudocubic phase coexistence zone, the x = 0.045 ceramics exhibited optimal piezoelectric and electromechanical coupling properties of d₃₃~121 pC/N and k_p~0.27 owing to decreased energy barriers for polarization switching. However, further addition of BNN could cause a decrease in freezing temperatures of polar nanoregions till the coexistence of nonergodic and ergodic relaxor phases occurred near room temperature, especially for the x = 0.05 sample which has negligible negative strains and thus show the maximum electrostrain of 0.3% under an external electric field of 7 kV/mm, but almost vanished piezoelectric properties. This was attributed to the fact that the induced long‐range ferroelectric order could reversibly switch back to its original ergodic state upon removal of external electric fields.     

Quenching-circumvented ergodicity in relaxor Na1/2Bi1/2TiO3-BaTiO3-K0.5Na0.5NbO3

Quenching alkaline bismuth titanates from sintering temperatures results in increased lattice distortion and consequently higher depolarization temperature. This work investigates the influence of quenching on the ergodicity of relaxor Na_1/2Bi_1/2TiO₃-BaTiO₃-K_0.5Na_0.5NbO₃. A distinct departure from ergodicity is evidenced from the increase in remanent polarization and the absence of frequency dispersion in the permittivity response of poled samples. Further, the samples exhibit enhanced negative strain upon application of electric field, indicating proclivity towards correlated polar nanoregions, corroborated by the enhanced tetragonal distortion. As a result, ergodic relaxor Na_1/2Bi_1/2TiO₃-6BaTiO₃-3K_0.5Na_0.5NbO₃ exhibits a depolarization temperature of 85°C with a 60% increase in remanent polarization and approximately a threefold increase in remanent strain upon quenching. Quenching-induced changes in the local environment of Na⁺ and Bi³⁺ cations hinder the development of ergodicity promoted by the A-site disorder. These results provide new insight into tailoring ergodicity of relaxor ferroelectrics.     

Electrical Properties of Lead‐Free Na1/2Bi1/2TiO3 Relaxor Ferroelectric Ceramics Doped with Hafnium or Zirconium

Ferroelectric (Na_1/2Bi_1/2)TiO₃ and its various solid solutions have been drawing special attention as a new candidate for their lead‐based counterparts. In this study, therefore, hafnium‐ or zirconium‐doped grain‐oriented (Na_1/2Bi_1/2)TiO₃ ceramics with <001>_pc orientation were fabricated by templated grain growth method using anisotropically shaped SrTiO₃ template particles. These molten salt synthesized SrTiO₃ platelets were tape cast with calcined (Na_1/2Bi_1/2)TiO₃ powders, and then sintered at 1200°C for 6 h. Texture fractions up to 70% have been obtained. Unipolar strains up to >0.25% were measured. Doped ceramics showed a “relaxor‐like” ferroelectric behavior. A broad dielectric peak with a slim hysteresis loop was present for hafnium‐ or zirconium‐doped samples. Electrostrictive coefficient of hafnium‐doped (Na_1/2Bi_1/2)TiO₃ ceramics were found to be 0.032 m⁴/C², which is much larger than that of PMN‐based electrostrictive materials. Electromechanical properties of hafnium‐ and zirconium‐doped (Na_1/2Bi_1/2)TiO₃ ceramics under electric bias were studied as well.     

High‐Temperature Multilayer Ceramic Capacitors Based on 100−x(94Bi1/2Na1/2TiO3–6BaTiO3)–xK0.5Na0.5NbO3

The potential high‐temperature dielectric materials 100?x(94Bi_1/2Na_1/2TiO₃–6BaTiO₃)–xK_0.5Na_0.5NbO₃ with x = 12, 18, and 24 were processed as bulk samples in order to examine the reduction of sintering temperature by means of CuO as sintering aid. Due to the successful reduction of sintering temperature, low cost Ag:Pd could be used as a co‐fired electrode material for multilayer ceramic capacitors (MLCCs). Fabrication of 8 μm thick, dense MLCCs with self‐contained, nonreactive electrodes is reported for a wide range of compositions of Bi_1/2Na_1/2TiO₃–BaTiO₃–K_0.5Na_0.5NbO₃. Among the manufactured MLCCs, those with compositions x = 24 showed the most promising dielectric properties for applications where high operating temperatures are needed. The temperature‐dependence of permittivity was quite low, revealing a change of less than ±10% compared to its 150°C‐value in the range of 40°C–225°C. For samples sintered at 1000°C, an RC constant of about 300 s was obtained at 150°C. Furthermore, these x = 24 MLCCs exhibited the finest microstructures among the compositions examined; making it a suitable candidate for further miniaturization of layer thickness as required for state‐of‐the art devices.     

Electrostatic energy storage performances of La(Ni2/3Ta1/3)O3-modified Na0.5Bi0.5TiO3 lead-free ceramics

Ceramic capacitors with high electrostatic energy storage performances have captured much research interest in latest years. Sodium bismuth titanate (Na_0.5Bi_0.5TiO₃)-based ferroelectric ceramics show great potential due to their environment-friendly composition, high polarization, and excellent relaxor properties. However, the nonergodic relaxor state of Na_0.5Bi_0.5TiO₃-based ceramics hampers the decrement of remanent polarization, leading to poor energy storage performance. Herein, the (1 − x)Na_0.5Bi_0.5TiO₃–xLa(Ni_2/3Ta_1/3)O₃ ceramics were designed to generate the transformation between nonergodic and ergodic relaxor state. As a result, the ceramics exhibit improved dielectric relaxation, slim polarization–electric field loops, and flattened current–electric field curves due to highly dynamic polar nanoregions. Particularly, the 0.85Na_0.5Bi_0.5TiO₃–0.15La(Ni_2/3Ta_1/3)O₃ ceramics show large breakdown electric field E_b (345 kV/cm), high recoverable energy density W_rec (3.6 J/cm³), and efficiency η (80.6%), revealing potential applications in electrostatic energy storage.     

Ferroelectric‐quasiferroelectric‐ergodic relaxor transition and multifunctional electrical properties in Bi0.5Na0.5TiO3‐based ceramics

A‐site substituted 0.88(Bi_0.5Na_0.5)_1?x(Li_0.5Nd_0.5)_xTiO₃–0.12BaTiO₃ (BNTLNx–BT12) ceramics were synthesized using a conventional solid‐state reaction route. The structural transformation and miscellaneous electrical properties were systematically investigated. The A‐site modification induced two sequence transitions from ferroelectric tetragonal (T) to quasi‐ferroelectric pseudocubic (PC) phase, followed closely by the second transition from non‐ergodic to ergodic relaxor (NR‐ER), and finally to dynamic polar nanoregions (PNRs). The significant enhancement in piezoelectric activity, strain response, broad plateau‐like maximum dielectric permittivity over a large temperature range and energy‐storage level at different compositions may be attributed to the compositionally‐induced T‐PC to NR‐ER transition and the alignment of dynamically‐fluctuating PNRs, respectively. The evolution of multifunctional electrical properties, associated with the variations in structure/microstructure, might provide a new insight to investigate the underlying mechanism of structure‐electrical properties relationship in ferroelectric solid solutions.