In situ study of electric-field-induced ferroelectric and antiferromagnetic domain switching in polycrystalline BiFeO3

Antiferromagnetic domain switching induced by ferroelectric polarization switching has previously been observed in situ in both multiferroic BiFeO₃ single crystals and thin films. Despite a number of reports on macroscopic magnetoelectric measurements on polycrystalline BiFeO₃, direct in situ observation of electric-field-induced antiferromagnetic domain switching in this material has not been addressed due to the lack of high-quality samples capable of electrical poling. Here, the electric field control of antiferromagnetic domain texture is identified in polycrystalline BiFeO₃ using in situ neutron diffraction, showing the resultant magnetic domain reorientation induced by an electric field. An antiferromagnetic domain reorientation to a value of 2.2-2.5 multiples of a random distribution (MRD) is found to be induced by an electric field that provides a non-180° ferroelectric-ferroelastic domain texture of 2.2-2.5 MRD along the field direction. The current results show well-controlled coupling of multiferroic domain texturing in single-phase polycrystalline BiFeO₃.     

Temperature dependence of field‐responsive mechanisms in lead zirconate titanate

An electric field loading stage was designed for use in a laboratory diffractometer that enables in situ investigations of the temperature dependence in the field response mechanisms of ferroelectric materials. The stage was demonstrated by measuring PbZr_1?xTi_xO₃ (PZT) based materials—a commercially available PZT and a 1% Nb‐doped PbZr_0.56Ti_0.44O₃ (PZT 56/44)—over a temperature range of 25°C to 250°C. The degree of non‐180° domain alignment (η₀₀₂) of the PZT as a function of temperature was quantified. η₀₀₂ of the commercially available PZT increases exponentially with temperature, and was analyzed as a thermally activated process as described by the Arrhenius law. The activation energy for thermally activated domain wall depinning process in PZT was found to be 0.47 eV. Additionally, a field‐induced rhombohedral to tetragonal phase transition was observed 5°C below the rhombohedral‐tetragonal transition in PZT 56/44 ceramic. The field‐induced tetragonal phase fraction was increased 41.8% after electrical cycling. A large amount of domain switching (η₀₀₂=0.45 at 1.75 kV/mm) was observed in the induced tetragonal phase.     

Enhancement of ferromagnetic and ferroelectric properties in calcium doped BiFeO3 by chemical synthesis

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), polarization and magnetic measurements. Structural studies by XRD and Rietveld refinement reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ (BFO) where enhanced ferroelectric and magnetic properties are produced by internal strain. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Fatigue and retention free characteristics are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain.     

Electromechanical properties of CaZrO3 modified (K,Na)NbO3‐based lead‐free piezoceramics under uniaxial stress conditions

Piezoelectric actuators are typically preloaded with a modest mechanical compressive stress during actuation to reduce cracking and allow for operation in the dynamic range. In addition, actuators are required to carry out mechanical work during operation, resulting in a nonlinear relationship between stress and actuation voltage. In fact, mechanical loading can significantly impact the electromechanical performance of lead‐free piezoelectrics. Herein, we report the dependence of electromechanical properties of CaZrO₃ modified (K,Na)NbO₃‐based lead‐free piezoceramics on uniaxial compressive stress, comparing to their lead‐based counterparts. It is demonstrated that increased non‐180° domain switching enhances the strain output at a moderate stress of approximately ?50 MPa from room temperature to 150°C. Larger uniaxial stress, however, is found to suppress ferroelectric domain switching, resulting in the continuous strain and polarization decrease.     

The Effect of Electric Poling on the Performance of Lead‐Free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 Piezoceramics

The effect of increasing poling fields on the properties of (1?x)BZT–xBCT compositions across the morphotropic phase boundary (MPB) is studied using large signal polarization and strain, small signal permittivity and piezoelectric coefficient, and XRD measurements. Successive poling causes charge carrier migration inducing an internal bias field, which becomes large with respect to the coercive field resulting in biased ferroelectric and ferroelastic switching. Improvements in piezoelectric coefficient of 9% are significantly smaller in the tetragonal 60BCT composition compared with the improvement of approximately 50% in the rhombohedral 40BCT and MPB 50BCT compositions. While the properties continue to change with increased poling fields, the remnant ferroelastic domain texture parallel to the field direction, as observed from XRD, stays approximately constant. The improvement in overall domain alignment leading to largely enhanced intrinsic piezoelectricity originates from the alignment of 180° domains and possibly non‐180° domains in grains with orientations inclined to the electric field. As a result, poling is most effective in BZT–BCT materials that have low coercive fields, show low distortions and possess more polarization orientations, such as compositions in the rhombohedral phase field or near the MPB.     

High Electric‐Induced Strain and Temperature‐Dependent Piezoelectric Properties of 0.75BF–0.25BZT Lead‐Free Ceramics

0.75BiFeO₃–0.25Ba(Zr_xTi_1?x) + 0.6 wt% MnO₂ (0.75BF–0.25BZT) ceramics with Mn addition were prepared by the solid‐state reaction method. The high‐field strain and high‐temperature piezoelectric properties of 0.75BF–0.25BZT ceramics were studied. Introduction of Zr in the solid solutions decreased the Curie temperature slightly, and improved the dielectric and piezoelectric properties obviously. The piezoelectric properties of 0.75BZT–0.25BT ceramics reached the maximum at Zr content of 10 mol%. The Curie temperature T_c, dielectric constant ε and loss tanδ (1 kHz), piezoelectric constant d₃₃, and planner electromechanical coupling factor k_p of 0.75BF–0.25BZT ceramics with 10 mol% Zr were 456°C, 650, 5%, 138 pC/N, and 0.30, respectively. The high‐field bipolar and unipolar strain under an electric field of 100 kV/cm reached up to 0.55% and 0.265%, respectively, which were comparable to those of BiScO₃–PbTiO₃ and “soft” PZT‐based ceramics. The typical “butterfly”‐shaped bipolar strain and frequency‐dependent peak‐to‐peak strain indicated that the large high‐field‐induced strain may be due to non‐180° domain switching. Rayleigh analysis reflected that the improved piezoelectric properties resulted from the enhanced extrinsic contribution by Zr doping. The unipolar strain of 0.75BF‐0.25BZT ceramics with 10 mol% Zr was almost linear from RT to 200°C. These results indicated that 0.75BF–0.25BZT ceramics were promising candidates for high‐temperature and lead‐free piezoelectric actuators.     

High strain (0.4%) Bi(Mg2/3Nb1/3)O3‐BaTiO3‐BiFeO3 lead‐free piezoelectric ceramics and multilayers

The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi(Mg_2/3Nb_1/3)O₃‐(0.95‐x)BaTiO₃‐xBiFeO₃ (BBFT, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics has been investigated. Scanning electron microscopy revealed a homogeneous microstructure for x < 0.75 but there was evidence of a core‐shell cation distribution for x = 0.75 which could be suppressed in part through quenching from the sintering temperature. X‐ray diffraction (XRD) suggested a gradual structural transition from pseudocubic to rhombohedral for 0.63 < x < 0.70, characterized by the coexistence of phases. The temperature dependence of relative permittivity, polarization‐electric field hysteresis loops, bipolar strain‐electric field curves revealed that BBFT transformed from relaxor‐like to ferroelectric behavior with an increase in x, consistent with changes in the phase assemblage and domain structure. The largest strain was 0.41% for x = 0.63 at 10 kV/mm. The largest effective piezoelectric coefficient (d₃₃^*) was 544 pm/V for x = 0.63 at 5 kV/mm but the largest Berlincourt d₃₃ (148 pC/N) was obtained for x = 0.70. We propose that d₃₃^* is optimized at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state but that d₃₃ is optimized in the ferroelectric, rhombohedral phase. Unipolar strain was measured as a function of temperature for x = 0.63 with strains of 0.30% achieved at 175°C, accompanied by a significant decrease in hysteresis with respect to room temperature measurements. The potential for BBFT compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved 3 μm displacement at 150°C.     

Thermally‐induced phase transformations in Na0.5Bi0.5TiO3–KNbO3 ceramics

The structures and functional properties of Na_0.5Bi_0.5TiO₃–xKNbO₃ (NBT‐xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high‐resolution synchrotron X‐ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long‐range ordered ferroelectric domains in the poled state. In contrast, the near‐cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of ?175°C. Upon heating, the field‐induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, T_ST, which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field‐induced rhombohedral phase in the poled specimens, the pseudo‐cubic lattice parameter, a_p, exhibited an anomalous reduction while the inter‐axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature‐dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature.     

Effect of BiMeO3 on the Phase Structure,Ferroelectric Stability,and Properties of Lead‐Free Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics

The effects of BiMeO₃ (Me = Fe, Sc, Mn, Al) addition on the phase transition and electrical properties of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT20) lead‐free piezoceramics were systematically investigated. Results showed that addition of BiFeO₃ into BNKT20 induces a phase transition from tetragonal–rhombohedral coexisted phases to a tetragonal phase with the observation of enhanced piezoelectric properties (d₃₃ = 150 pC/N for 0.02BiFeO₃). BiScO₃, BiMnO₃, and BiAlO₃ substitutions into BNKT20 induce a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic 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, large accompanying normalized strains of 0.34%–0.36% are obtained near the ferroelectric–relaxor phase boundary, and the mergence of large strain response can be ascribed to a reversible field‐induced ergodic relaxor‐to‐ferroelectric phase transformation. Moreover, our study also revealed that the composition located at the ferroelectric–relaxor phase boundary where the strain response is consistently derivable shifts to a BNKT20‐rich composition as the tolerance factor t of the end‐member BiMeO₃ increases, and this relationship is expected to provide a guideline for designing high‐performance (Bi_0.5Na_0.5)TiO₃‐based materials by searching the ferroelectric–relaxor phase boundary.     

Manganese‐Doped BiFeO3–BaTiO3 High‐Temperature Piezoelectric Ceramics: Phase Structures and Defect Mechanism

The bulk BiFeO₃–BaTiO₃ system has been studied as a potential lead‐free high‐temperature piezoelectric material. It is found that the multivalency manganese‐doped 0.8BiFeO₃–0.2BaTiO₃ ceramics exhibit the coexistence of tetragonal and rhombohedral phases, whereas Mn doping improves the resistivity and exhibits hard characteristic. The optimal properties were obtained at 0.12 wt% Mn addition exhibiting T_c ? 637°C. The increase of T_c in Mn‐modified compositions can be ascribed to the appearance of internal electric bias field by acceptor doping. The combination of good electrical properties and high T_c makes these ceramics suitable for elevated temperature piezoelectric devices.     

A Novel BiFeO3–BaTiO3–BaZrO3 Lead‐Free Relaxor Ferroelectric Ceramic with Low‐Hysteresis and Frequency‐Insensitive Large Strains

A novel (0.67?x)BiFeO₃–0.33BaTiO₃–xBaZrO₃ lead‐free relaxor ferroelectric ceramic was developed by a solid‐state reaction method. Measurements of temperature‐dependent dielectric permittivity and the polarization/strain hysteresis loops demonstrated an obvious evolution of dielectric relaxor behavior at room temperature (RT) from nonergodic to ergodic states. A significantly enhanced electrostrain of ~0.37% at 7 kV/mm with a relatively small hysteresis of ~39% and a low‐frequency sensitivity was found at x = 0.04, showing large potential for actuator applications. This was basically attributed to a rapid response of forward and backward switching between ergodic and ferroelectric phases owing to similar free energies and large local random fields.     

Anisotropic domain switching in Pb(Mg1/3Nb2/3)O3‐0.30PbTiO3 single crystals with rhombohedral structure

Anisotropic domain switching paths in [001]‐, [011]‐, and [111]‐poled Pb(Mg_1/3Nb_2/3)O₃‐0.30PbTiO₃ single crystals were studied by in situ polarized light microscopic driven by an antiparallel electric field. Orientation‐dependent electric field induced polarization and strain behaviors were investigated systematically. For [001]‐oriented crystals, only one‐step 71° switching occurred during the domain switching process, resulting in the appearance of stripe domain walls whose traces on (001) plane were along 45° or 135° with respect to [100] direction. But for [011]‐oriented samples, a two‐step 71° switching was observed during 109° switching and the projections of formed twin domain walls on the (011) plane are along 35.3° or 144.7° with respect to [01] direction. Moreover, a three‐step 71° switching was found during 180° switching in [111]‐oriented samples. It was demonstrated by the produced domain walls whose projections on the (10) plane are along 35.3°, 90° or 160.6° with respect to [11] direction. The energetically motivated mechanism based on multistep polarization switching process was also proposed to explain the anisotropic domain switching paths. Our results provided a visualized observation on the ferroelectric domain switching process and also laid the solid foundations for controlling polarization order parameter in ferroelectric single crystals.     

Uniaxial compressive stress and temperature dependent mechanical behavior of (1-x)BiFeO3-xBaTiO3 lead-free piezoelectric ceramics

The mechanical behavior of polycrystalline lead-free (1-x)BiFeO₃-xBaTiO₃ (BF-BT) piezoelectric ceramics was investigated under uniaxial compressive stress from room temperature up to 400 °C with macroscopic stress-strain measurements and in situ stress-dependent neutron diffraction. Stress-strain curves revealed a changing mechanical response with BaTiO₃ content and temperature. With decreasing BaTiO₃ content there was an increase in the coercive stress, which reduced the remanent strain and hysteresis. Full pattern structural refinement of the neutron data reveals both rhombohedral distortion and magnetic moment decreases with increasing BaTiO₃ content. In situ stress-dependent neutron diffraction experiments showed that accommodation of external stress occurs through the changes in tilt magnitude and anisotropy of oxygen octahedra at room temperature. The origin of stress-induced strain at room temperature is a lattice deformation without any apparent change in average crystallographic symmetry or domain switching. Temperature-dependent in situ stress-induced measurement of BF-30BT showed maximum strain close to the rhombohedral - pseudocubic transition temperature, which has been proposed to be due to the lattice deformation as well as to the differing degree of tilting of the (Fe/Ti)O₆ octahedra.     

Temperature Gradient Introduced Ferroelectric Self‐Poling in BiFeO3 Ceramics

The as‐prepared BiFeO₃ ceramic shows a piezoelectric d₃₃ coefficient of ?14 pC/N, that is, an obvious ferroelectric self‐poling phenomenon. The temperature gradient between the two surfaces of BiFeO₃ ceramic was intentionally enlarged when BiFeO₃ was prepared with a rapid liquid sintering method. This temperature gradient and the corresponding thermal strain can introduce defect dipoles through separating bismuth vacancies from oxygen vacancies. A mass of these dipoles introduce a macroscopic internal electric field (E_in) which downward poles BiFeO₃ ceramic during its cooling down process. As expected, an E_in of >10 kV/cm is confirmed by the asymmetrical polarization/strain versus electric field curves.     

Anomalously large lattice strain contributions from rhombohedral phases in BiFeO3-based high-temperature piezoceramics estimated by means of in-situ synchrotron x-ray diffraction

Thermally-stable (0.75-x)BiFeO₃-0.25PbTiO₃-xBa(Zr_0.25Ti_0.75)O₃ (0.1?≤?x?≤?0.27) piezoelectric ceramics were reported to have excellent dielectric and electromechanical properties of d₃₃～405 pC/N, k_p～46%, ε₃₃^T/ε₀～1810, tanδ～3.1% and T_c～421?°C close to tetragonal (T)-rhombohedral (R) morphotropic phase boundary. The dielectric measurement indicates that R ferroelectric phase is gradually transformed into relaxor ferroelectric across the phase boundary due to the substitution of BZT for BF. The transmission electron microscopy and convergent beam electron diffraction provide clear evidences that both the R-T phase coexistence and polar nanodomains contribute to enhanced piezoelectric properties at x?=?0.19 through cooperatively facilitating polarization orientation. In combination with the macroscopic piezoelectric coefficient measurement, the quantitative analysis of synchrotron diffraction data under electric fields suggests that extremely large lattice strain contribution predominantly from R phases plus little extrinsic domain switching contribution should dominate the piezoelectric response of the x?=?0.19 sample, mainly owing to both irreversible field-induced T to R phase transition and irreversible non-180° domain switching.     

Electromechanical Response of Polycrystalline Barium Titanate Resolved at the Grain Scale

Ferroic materials are critical components in many modern devices. Polycrystalline states of these materials dominate the market due to their cost effectiveness and ease of production. Studying the coupling of ferroic properties across grain boundaries and within clusters of grains is therefore critical for understanding bulk polycrystalline ferroic behavior. Here, three‐dimensional X‐ray diffraction is used to reconstruct a 3D grain map (grain orientations and neighborhoods) of a polycrystalline barium titanate sample and track the grain‐scale non‐180° ferroelectric domain switching strains of 139 individual grains in situ under an applied electric field. The map shows that each grain is located in a very unique local environment in terms of intergranular misorientations, leading to local strain heterogeneity in the as‐processed state of the sample. While primarily dependent on the crystallographic orientation relative to the field directions, the response of individual grains is also heterogeneous. These unique experimental results are of critical importance both when building the starting conditions and considering the validity of grain‐scale modeling efforts, and provide additional considerations in the design of novel ferroic materials.     

In Situ Domain Structure Observation and Giant Magnetoelectric Coupling for PMN‐PT/Terfenol‐D Multiferroic Composites

In situ observations of ferroelectric domain structure evolution, and magnetoelectric (ME) coupling are investigated for PMN‐28PT/Terfenol‐D (abbreviation of Pb(Mg_1/3Nb_2/3)O₃‐28PbTiO₃/Tb_0.3Dy_0.7Fe₂) and PMN‐33PT/Terfenol‐D composites under the magnetic loadings. The composite of PMN‐33PT/Terfenol‐D shows stronger ME coupling than that in PMN‐28PT/Terfenol‐D. At a thickness of 0.10–0.12 mm for the single crystal plate, a giant magnetoelectric coefficient (α_ME) up to 2 V/cm·Oe is obtained for PMN‐33PT/Terfenol‐D at a static magnetic field of 200 Oe and 1 kHz of the alternating magnetic field. In situ domain structure observations reveal the domain morphology change during the applied magnetic loadings. In PMN‐28PT, the domains are of predominantly rhombohedral (R) phase and they change into monoclinic M_A phase upon the magnetic loading via the strain transferred between Terfenol‐D plate and PMN‐PT single crystal. In PMN‐33PT, domains of orthorhombic (O), R, and monoclinic M_C coexist and phase transitions from O to M_C and further to R phase occur upon the magnetic loading. The undulation and diversity of the domain structure makes the domains more susceptible to the magnetic loading via strain transferred between Terfenol‐D plate and PMN‐PT single crystal, and consequently, a strong ME coupling in the composites.     

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.     

Strain effects on domain structures in ferroelectric thin films from phase‐field simulations

Strain and applied external electric fields are known to influence domain evolution and associated ferroelectric responses in ferroelectric thin films. Here, phase‐field simulations are used to predict equilibrium domain structures and polarization‐field (P‐E) hysteresis loops of lead zirconate titanate (PZT) thin films under a series of mismatch strains, ranging from strongly tensile to strongly compressive. In particular, the evolution of domains and the P‐E curves under different applied strains reveal the mesoscale mechanism, the appearance of in‐plane polarization during domain switching, that is responsible for a relatively small coercive field and remnant polarization. A Landau energy distribution is analyzed to better understand the domain evolution under various strain conditions. The results provide guidance for choice of mismatched strains to yield the desired P‐E hysteresis loops and the domain structures.     

Impact of fatigue behavior on energy storage performance in dielectric thin-film capacitors

The polarization hysteresis loops and the dynamics of domain switching in ferroelectric Pb(Zr_0.52Ti_0.48)O₃ (PZT), antiferroelectric PbZrO₃ (PZ) and relaxor-ferroelectric Pb_0.9La_0.1(Zr_0.52Ti_0.48)O₃ (PLZT) thin films deposited on Pt/Ti/SiO₂/Si substrates were investigated under various bipolar electric fields during repetitive switching cycles. Fatigue behavior was observed in PZT thin films and was accelerated at higher bipolar electric fields. Degradation of energy storage performance observed in PZ thin films corresponds to the appearance of a ferroelectric state just under a high bipolar electric field, which could be related to the nonuniform strain buildup in some regions within bulk PZ. Meanwhile, PLZT thin films demonstrated fatigue-free in both polarization and energy storage performance and independent bipolar electric fields, which are probably related to the highly dynamic polar nanodomains. More importantly, PLZT thin films also exhibited excellent recoverable energy-storage density and energy efficiency, extracted from the polarization hysteresis loops, making them promising dielectric capacitors for energy-storage applications.