Tailoring high-energy storage NaNbO3-based materials from antiferroelectric to relaxor states.

Reversible field-induced phase transitions define antiferroelectric perovskite oxides and lay the foundation for high-energy storage density materials, required for future green technologies. However, promising new antiferroelectrics are hampered by transition´s irreversibility and low electrical resistivity. Here, we demonstrate an approach to overcome these problems by adjusting the local structure and defect chemistry, delivering NaNbO3-based antiferroelectrics with well-defined double polarization loops. The attending reversible phase transition and structural changes at different length scales are probed by in situ high-energy X-ray diffraction, total scattering, transmission electron microcopy, and nuclear magnetic resonance spectroscopy. We show that the energy-storage density of the antiferroelectric compositions can be increased by an order of magnitude, while increasing the chemical disorder transforms the material to a relaxor state with a high energy efficiency of 90%. The results provide guidelines for efficient design of (anti-)ferroelectrics and open the way for the development of new material systems for a sustainable future.

Fundamental workings of chemical substitution at the A-site of perovskite oxides- a 207Pb NMR study of Ba-substituted PbZrO3.

Lead zirconate (PbZrO3, PZ) is a prototype antiferroelectric (AFE) oxide from which state-of-the-art energy storage materials are derived by chemical substitutions. A thorough understanding of the structure-property relationships of PZ-based materials is essential for both performance improvement and the design of more environmentally friendly replacements. (Pb1-xBax)ZrO3 (PBZ) can serve as a model system for studying the effect of A-site substitution in the perovskite lattice, with barium destabilizing the AFE state. Here, the two-dimensional 207Pb solid-state NMR spectra of PZ and PBZ were recorded to analyze the local structural role of barium substitution. At low substitution levels, 207Pb NMR spectroscopy reveals the presence of Pb-O bond length disorder. Upon crossing the threshold value of x for the macroscopic phase transition into a ferroelectric (FE) state, the barium cations cause local-scale lattice expansions in their vicinity, resulting in the collapse of two lead lattice sites into one. The stabilization of the larger volume site coincides with the favoring of larger lead displacements. We also observed more covalent bonding environments which may originate from the lower polarizability of the barium cations, facilitating the formation of stronger Pb-O bonds in their vicinity. From the local structural point of view, we propose that the substitution-induced AFE → FE phase transition is therefore related to an increasing correlation of larger lead displacements in larger oxygen cavities as the barium content increases. Our results also highlight 207Pb NMR spectroscopy as a valuable method for the characterization of the structure-property relationships of PbZrO3-based AFE and FE oxides.

Anisotropic dislocation-domain wall interactions in ferroelectrics.

Dislocations are usually expected to degrade electrical, thermal and optical functionality and to tune mechanical properties of materials. Here, we demonstrate a general framework for the control of dislocation-domain wall interactions in ferroics, employing an imprinted dislocation network. Anisotropic dielectric and electromechanical properties are engineered in barium titanate crystals via well-controlled line-plane relationships, culminating in extraordinary and stable large-signal dielectric permittivity (≈23100) and piezoelectric coefficient (≈2470 pm V-1). In contrast, a related increase in properties utilizing point-plane relation prompts a dramatic cyclic degradation. Observed dielectric and piezoelectric properties are rationalized using transmission electron microscopy and time- and cycle-dependent nuclear magnetic resonance paired with X-ray diffraction. Succinct mechanistic understanding is provided by phase-field simulations and driving force calculations of the described dislocation-domain wall interactions. Our 1D-2D defect approach offers a fertile ground for tailoring functionality in a wide range of functional material systems.

Coherent Precipitates with Strong Domain Wall Pinning in Alkaline Niobate Ferroelectrics.

High-power piezoelectric applications are predicted to share approximately one-third of the lead-free piezoelectric ceramic market in 2024 with alkaline niobates as the primary competitor. To suppress self-heating in high-power devices due to mechanical loss when driven by large electric fields, piezoelectric hardening to restrict domain wall motion is required. In the present work, highly effective piezoelectric hardening via coherent plate-like precipitates in a model system of the (Li,Na)NbO3 (LNN) solid solution delivers a reduction in losses, quantified as an electromechanical quality factor, by a factor of ten. Various thermal aging schemes are demonstrated to control the average size, number density, and location of the precipitates. The established properties are correlated with a detailed determination of short- and long-range atomic structure by X-ray diffraction and pair distribution function analysis, respectively, as well as microstructure determined by transmission electron microscopy. The impact of microstructure with precipitates on both small- and large-field properties is also established. These results pave the way to implement precipitate hardening in piezoelectric materials, analogous to precipitate hardening in metals, broadening their use cases in applications.

Measurement System for Piezoelectric Resonance Impedance Spectroscopy Under Combined AC and High-Voltage DC Loading.

Piezoelectric resonance impedance spectroscopy is a standardized measurement technique for determining the electromechanical, elastic, and dielectric parameters of piezoceramics. However, commercial measurement setups are designed for small-signal measurements and encounter difficulties when constant driving voltages/currents are required at resonances, higher fields, or combined AC and DC loading. The latter is particularly important to evaluate the DC bias-hardening effect of piezoelectrics. Here, we propose a novel measurement system for piezoelectric resonance impedance spectroscopy under combined AC and high-voltage DC loading that complies with established standards. The system is based on two separate output amplifier stages and includes voltage/current probes, a laser vibrometer, custom protection components, and control software with optimization algorithm. In its current form, the measurement setup allows the application of AC frequencies up to 500 kHz and DC signals up to ±10 kV on samples with impedance between 10-1 and 106 Ω . The operation of the proposed setup was benchmarked against commercial impedance analyzers in the small-signal range and reference equivalent circuits. Test measurements under combined AC and DC loading were performed on a soft Pb(Zr,Ti)O3 piezoceramic. The results revealed that a DC bias voltage applied along the polarization direction ferroelectrically hardens the material, while the material softens and eventually depolarizes when the DC bias voltage is applied in the opposite direction. The results confirm the suitability of the designed measurement system and open new exciting possibilities for tuning the piezoelectric properties by DC bias fields.

Precipitation Hardening in Ferroelectric Ceramics.

Domain wall motion in ferroics, similar to dislocation motion in metals, can be tuned by well-concepted microstructural elements. In demanding high-power applications of piezoelectric materials, the domain wall motion is considered as a lossy hysteretic mechanism that should be restricted. Current applications for so-called hard piezoelectrics are abundant and hinge on the use of an acceptor-doping scheme. However, this mechanism features severe limitations due to enhanced mobility of oxygen vacancies at moderate temperatures. By analogy with metal technology, the authors present here a new solution for electroceramics, where precipitates are utilized to pin domain walls and improve piezoelectric properties. Through a sequence of sintering, nucleation, and precipitate growth, intragranular precipitates leading to a fine domain structure are developed as shown by transmission electron microscopy, piezoresponse force microscopy, and phase-field simulation. This structure impedes the domain wall motion as elucidated by electromechanical characterization. As a result, the mechanical quality factor is increased by ≈50% and the hysteresis in electrostrain is suppressed considerably. This is even achieved with slightly increased piezoelectric coefficient and electromechanical coupling factor. This novel process can be smoothly implemented in industrial production processes and is accessible to simple laboratory experimentation for microstructure optimization and implementation in various ferroelectric systems.

Control of polarization in bulk ferroelectrics by mechanical dislocation imprint.

Defects are essential to engineering the properties of functional materials ranging from semiconductors and superconductors to ferroics. Whereas point defects have been widely exploited, dislocations are commonly viewed as problematic for functional materials and not as a microstructural tool. We developed a method for mechanically imprinting dislocation networks that favorably skew the domain structure in bulk ferroelectrics and thereby tame the large switching polarization and make it available for functional harvesting. The resulting microstructure yields a strong mechanical restoring force to revert electric field-induced domain wall displacement on the macroscopic level and high pinning force on the local level. This induces a giant increase of the dielectric and electromechanical response at intermediate electric fields in barium titanate [electric field-dependent permittivity (ε33) ≈ 5800 and large-signal piezoelectric coefficient (d 33*) ≈ 1890 picometers/volt]. Dislocation-based anisotropy delivers a different suite of tools with which to tailor functional materials.

Polarization Rotation at Morphotropic Phase Boundary in New Lead-Free Na1/2Bi1/2V1-xTixO3 Piezoceramics.

In this work, we show that polarization rotation enhances the piezoresponse in a high-performance lead-free piezoelectric material, Na1/2Bi1/2V1-xTixO3, a solid solution between tetragonal Na1/2Bi1/2VO3 and rhombohedral Na1/2Bi1/2TiO3, obtained by high-pressure synthesis. The system forms a pure perovskite structure with a favorable morphotropic phase boundary (MPB) located around x = 0.90, which separates the tetragonal and rhombohedral phases. In addition, a distinct monoclinic phase with polarization rotation as functions of composition and temperature is observed. XRD measurements revealed the moderately high Curie temperature of 523 K at x = 0.95 in the MPB. The piezoelectric coefficient d33 of the monoclinic x = 0.95 sample, 42 pC/N, is higher than those of the tetragonal and rhombohedral phases. Even though the present lead-free Na1/2Bi1/2V1-xTixO3 ceramics feature smaller d33 values compared to many currently available lead-free piezoelectric materials as a result of insufficient poling and low density, we expect our findings open up opportunities for exploring promising lead-free piezoelectric materials in Na1/2Bi1/2VO3-based perovskites.

Ferroelastic Properties of PZT: Characterization Under Compressive and Tensile Stress, Finite-Element Simulation, and Lifetime Calculation.

Finite-element (FE) simulations, based on a phenomenological nonlinear constitutive model, were carried out to show the impact of ferroelastic switching on compressive and tensile loading of a soft and hard ferroelastic polycrystalline lead zirconate titanate ceramic (PbZr1-xTixO3, PZT). Hereby, a model assembly is used that simulates mechanical stresses generated by the contact of a ceramic element with a concave counterpart. In order to parameterize the constitutive model, the elastic and piezoelectric tensors are determined by the small signal resonance method and by the direct measurement under compressive/tensile loads. In addition, the large field ferroelectric and ferroelastic behavior of polarization and strain were characterized. The simulations show that the tensile stress level within the piezoceramic is much lower compared to a linear elastic model, which is related to ferroelastic switching. This is demonstrated to have a significant influence on mechanical lifetime predictions. To validate the parametrized FE model, a four-point bending experiment was used. The comparison of the calculated and measured strain on the compressive and tensile sides of the sample showed very good correlation.

Temperature-Dependent Evolution of Crystallographic and Domain Structures in (K,Na,Li)(Ta,Nb)O3 Piezoelectric Single Crystals.

(K,Na)NbO3-based ferroelectric single crystals have recently undergone a substantial development, resulting in improved crystal quality and large piezoelectric coefficients, exceeding 700 pC/N, over a broad temperature range. However, further development necessitates a detailed understanding of the mechanisms defining the domain structure and its temperature evolution. This paper presents the investigation into the crystallographic structure and domain configurations of a (K,Na,Li)(Ta,Nb)O3 single crystal over a broad temperature range. The crystal was grown by the submerged-seed solution growth technique and investigated using in situ transmission electron microscopy, X-ray diffraction, dielectric measurements, and polarized light microscopy. The lattice distortion, structural phase transitions, and domain configurations are reported. A transition from the lamellar orthorhombic to the rectangular tetragonal domain structure is observed upon heating. Moreover, the milky optical appearance of the crystal was investigated and found to result from the presence of regions with different domain configurations and domain sizes. The formation of these regions is related to the growth defects, which govern the domain formation when cooling below the Curie temperature.

Inverted electro-mechanical behaviour induced by the irreversible domain configuration transformation in (K,Na)NbO3-based ceramics.

Miniaturization of domains to the nanometer scale has been previously reported in many piezoelectrics with two-phase coexistence. Despite the observation of nanoscale domain configuration near the polymorphic phase transition (PPT) regionin virgin (K0.5Na0.5)NbO3 (KNN) based ceramics, it remains unclear how this domain state responds to external loads and influences the macroscopic electro-mechanical properties. To this end, the electric-field-induced and stress-induced strain curves of KNN-based ceramics over a wide compositional range across PPT were characterized. It was found that the coercive field of the virgin samples was highest in PPT region, which was related to the inhibited domain wall motion due to the presence of nanodomains. However, the coercive field was found to be the lowest in the PPT region after electrical poling. This was related to the irreversible transformation of the nanodomains into micron-sized domains during the poling process. With the similar micron-sized domain configuration for all poled ceramics, the domains in the PPT region move more easily due to the additional polarization vectors. The results demonstrate that the poling process can give rise to the irreversible domain configuration transformation and then account for the inverted macroscopic piezoelectricity in the PPT region of KNN-based ceramics.

Enhancing electromechanical properties of lead-free ferroelectrics with bilayer ceramic/ceramic composites.

The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor, (Bi1/2Na1/2)TiO3-7BaTiO3, with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi1/2Na1/2)TiO3-25SrTiO3] that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain- and polarization-electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.

Sintering of Lead-Free Piezoelectric Sodium Potassium Niobate Ceramics.

The potassium sodium niobate, K0.5Na0.5NbO3, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).

Electric-field-induced phase transitions in co-doped Pb(Zr1-xTix)O3 at the morphotropic phase boundary.

The strain- and polarization-electric field behavior was characterized at room temperature for Pb0.98Ba0.01(Zr1-x Ti x )0.98Nb0.02O3, 0.40 ⩽ x ⩽ 0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.