Effect of thermal depolarization on the poling‐induced domain texture and piezoelectric properties in Mg‐doped NBT‐6BT

Recently, poled Na₀.₅₀Bi₀.₅₀TiO₃‐BaTiO₃ (NBT‐BT)‐based polycrystalline materials have been characterized as possessing a high degree of poling‐induced domain texture in their remanent state. This finding is suggested to be the reason for their stable mechanical quality factor at high‐vibration velocity, making them promising candidates for high‐power applications. The materials in consideration are prone to self‐heating and thermal run‐away, particularly at slightly elevated temperatures. Therefore, this paper evaluates the temperature dependence of the poling‐induced domain texture of (Na₀.₄₇Bi₀.₄₇Ba₀.₀₆)TiO₃ (NBT‐6BT) doped with 0.5 mol% of Mg as compared to undoped NBT‐6BT. Its influence on small‐signal, large‐signal, and high‐power properties was investigated. To obtain a fundamental understanding of crystal structure, in‐situ synchrotron measurements were conducted as function of temperature to establish a relationship between structure and piezoelectric properties of both Mg‐doped and undoped NBT‐6BT materials

Re-entrant relaxor ferroelectric behaviour in Nb-doped BiFeO 3 –BaTiO 3 ceramics †

BiFeO3–BaTiO3 (BF–BT) solid solutions exhibit great promise as the basis for high temperature piezoelectric transducers and energy storage dielectrics, but the fundamental mechanisms governing their functional properties require further clarification. In the present study, both pure and niobium-doped 0.7BF–0.3BT ceramics are synthesized by solid state reaction and their structure–property relationships are systematically investigated. It is shown that substituting a low concentration of Ti with Nb at a level of 0.5 at% increases the resistivity of BF–BT ceramics and facilitates ferroelectric switching at high electric field levels. Stable planar piezoelectric coupling factor values are achieved with a variation from 0.35 to 0.45 over the temperature range from 100 to 430 °C. In addition to the ferroelectric-paraelectric phase transformation at the Curie point (∼430 °C), a frequency-dependent relaxation of the dielectric permittivity and associated loss peak are observed over the temperature range from −50 to +150 °C. These effects are correlated with anomalous enhancement of the remanent polarization and structural (rhombohedral) distortion with increasing temperature, indicating the occurrence of a re-entrant relaxor ferroelectric transformation on cooling. The results of the study provide new insight into the thermal evolution of structure and the corresponding functional properties in BF–BT and related solid solutions

Quenching-Induced Changes in the Structural and Electrical Properties of Lead-Free Ferroelectric Ceramics

Ferroelectrics are an important class of functional materials utilized in various electronic devices, often in the form of polycrystalline ceramics. The most commonly used ferroelectric ceramics are lead-containing Pb(Zr,Ti)O3-based materials. However, due to the toxicity of lead and the resulting environmental concerns, the EU legislation on the Restriction of Hazardous Substances has emphasized the need to identify lead-free alternatives. Na1/2Bi1/2TiO3-BaTiO3 (NBT-BT) and BiFeO3-BaTiO3 (BF-BT) solid solutions are among the promising lead-free candidates for high power and high temperature applications, respectively. Both compositions exhibit certain drawbacks, with NBT-BT having a low temperature stability and BF-BT rather low piezoelectric coefficient (d33) and resistivity. These disadvantages can be mitigated to some extent by quenching the material from high temperatures. The present study aims to investigate the quenching-induced changes in the structural and electrical characteristics of lead-free Bi-based ceramics to advance the understanding of the underlying mechanisms. The investigated materials are NBT-BT with 3, 6, 9 and 12 mol% BT, and BF-BT with 30 and 33 mol% BT with and without the addition of 5 mol% NBT. High-resolution X-ray powder diffraction and synchrotron diffraction experiments on bulk samples confirmed that quenching increases the lattice distortion in NBT-BT. This effect is particularly pronounced for 6 mol% BT, located at the morphotropic phase boundary (MPB). For this composition, which is a non-ergodic relaxor at room temperature, quenching alters the phase fractions, leading to a notable decrease in the volume of the cubic phase. In the poled state, the lattice distortion for the quenched samples is more pronounced in comparison to the furnace cooled counterparts, presumably being the reason for the increased depolarization temperature, Td. In situ electric field-dependent synchrotron diffraction revealed a shift in the onset of electric field-induced structural changes to higher electric field amplitudes. Further, quenched NBT-BT exhibits strongly reduced volumetric strain compared to the furnace cooled sample. This can be attributed to the stabilized ferroelectric domain state, which limits the possibility for quenching-induced phase transformation. The increase in lattice distortion and spontaneous strain in NBT-BT upon quenching is also reflected in increased polarization, coercive field and total strain as established from ferroelectric hysteresis measurements. However, quenching decreases the d33 in all the investigated NBT-BT compositions. Dielectric characterization revealed an increase in the ferroelectric to relaxor transition temperature, and thus, Td. To elucidate the influence of VI quenching on the nanoscale structure of NBT-BT at elevated temperatures, temperature-dependent Young’s modulus and a composite model were used to calculate the volume fraction of polar nanoregions (PNRs). Above the temperature of maximum permittivity, quenching causes an increase in PNR volume fraction at and close to the MPB, likely due to the enhanced non-cubic distortions, which is consistent with the change in phase composition at room temperature. The addition of 5 mol% NBT into BF-BT with 30 and 33 mol% BT promotes a more cubic structure with a smaller rhombohedral distortion and lower rhombohedral phase fraction. Furthermore, the ternary composition exhibits significantly reduced dielectric losses. BF-BT-NBT demonstrates characteristics of a partially ergodic relaxor material in its ferroelectric response, displaying low remanent polarization and strain, which can be correlated to a decrease in d33 values. Quenching BF-BT-NBT leads to vanishing of the ergodicity and enhanced negative strain and remanent polarization. The d33 value is increased by up to 107 %, and the rhombohedral lattice distortion is enhanced upon quenching. In both investigated material systems, quenching induces changes in structural and electrical characteristics, yielding beneficial effects on application-relevant properties. The enhanced lattice distortion in quenched NBT-BT results in an increased Td, improving the temperature-range for applications. Moreover, the Young’s modulus is only marginally lower compared to the furnace cooled material, indicating insignificant deterioration of the mechanical properties. In quenched BF-BT-NBT, the d33 value is greatly improved, and the increase in resistivity upon introducing NBT into BF-BT is maintained. These findings emphasize that quenching Bi-based piezoceramics can help bringing these lead-free materials into application

In situ hot-stage TEM of the phase and domain evolution in quenched Na1/2Bi1/2TiO3–BaTiO3

Quenching relaxor ferroelectric 0.94(Na1/2Bi1/2)TiO3–0.06BaTiO3(NBT-6BT)enhances the depolarization temperature (Td), linked to the stabilization offerroelectric order. The thermal evolution of the domain structure and phaseassemblage provides insights about the onset of ferroelectric order in quenchedmaterials. Unpoled furnace cooled and quenched NBT-6BT ceramics were stud-ied using in situ temperature-dependent transmission electron microscopy. Therhombohedraltotetragonalstructuraltransitioninfurnacecooledandquenchedsamples occurs in a comparable temperature range of 120◦C–220◦C. While thetetragonal phase is characterized by polar nanoregions (PNRs) and no domaincontrast in the furnace cooled state, the quenched composition exhibits anincreased fraction of lamellar domains, which are partially stable up to 300◦C,thus benefiting the delayed depolarization. This is further corroborated by the dielectric data indicating earlier freezing of PNR dynamics in the quenched state.The reversibility of the phase transition is demonstrated by successive cooling,where quenched NBT-6BT features an increased grainy PNR contrast after theexperiment, followed by a kinetically delayed coalescence of PNRs back intolamellar domains. This demonstrates that the stabilized ferroelectric state uponquenching is associated with the conversion of polar units on the nanometerscale into long-range domain structures

Nanoscale polar regions embedded within ferroelectric domains in Na1/2Bi1/2TiO3–BaTiO3

Relaxor ferroelectrics are an eminent group of functional materials, characterized by complex micro- and nanoscale structures, accounting for their enhanced piezoelectric properties. In the (1−x)Na 1/2 Bi 1/2 TiO3 -xBaTiO3 (NBT-BT) solid solution, the evolution of nanoscale domains and their hierarchical association with ferroelectric domains is investigated using conventional and scanning transmission electron microscopy on compositions with 6, 9, and 12 mol % BT. Short-range fluctuations in the local polar displacement (polar nanoregions, PNRs) account for a heterogeneous nanostructure at the morphotropic phase boundary (6 mol % BT). Platelike nanodomains of tetragonal P4bm symmetry coexist with a minor volume fraction of rhombohedral R3c nanodomains. Their overall population decreases with increasing BT content. However, ferroelectric P4mm domains in the composition with 12 mol % BT still exhibit nanoscale regions, which deviate from the average polarization. Small volume fractions of both P4bm and R3c nanodomains remain embedded within the ferroelectric domains. This hierarchical domain configuration underpins the complex structural characteristics of NBT-based relaxor ferroelectrics

Effect of thermal depolarization on the poling-induced domain texture and piezoelectric properties in Mg-doped NBT-6BT

Recently, poled Na0.50Bi0.50TiO3-BaTiO3 (NBT-BT)-based polycrystalline materials have been characterized as possessing a high degree of poling-induced domain texture in their remanent state. This finding is suggested to be the reason for their stable mechanical quality factor at high-vibration velocity, making them promising candidates for high-power applications. The materials in consideration are prone to self-heating and thermal run-away, particularly at slightly elevated temperatures. Therefore, this paper evaluates the temperature dependence of the poling-induced domain texture of (Na0.47Bi0.47Ba0.06)TiO3 (NBT-6BT) doped with 0.5 mol% of Mg as compared to undoped NBT-6BT. Its influence on small-signal, large-signal, and high-power properties was investigated. To obtain a fundamental understanding of crystal structure, in-situ synchrotron measurements were conducted as function of temperature to establish a relationship between structure and piezoelectric properties of both Mg-doped and undoped NBT-6BT materials

Texture-based ferroelectric hardening in Na1/2_{1/2}Bi1/2_{1/2}TiO3_3 -based piezoceramics

Na$_{1/2}$Bi$_{1/2}$TiO$_3$-based (NBT-based) ceramics offer a viable option to replace lead-based materials for high-power applications as they are characterized by a stable mechanical quality factor with increasing vibration velocity in comparison to lead-based piezoceramics. Recently, the minor and stable extrinsic contributions were revealed as the origin for the stability of the mechanical quality factor with increasing vibration velocity. This work identifies the very unusual high poling degree as cause for the small extrinsic contributions. To this end, complete pole figure densities have been quantified and correlated to the piezoelectric coefficient and electromechanical quality factor. This hypothesis is further strengthened by correlating the piezoelectric constant (sum of intrinsic and extrinsic contributions) with the remanent polarization (correlates to remanent texturing degree). In order to assess a full picture of NBT-based piezoceramics, 0.94Na$_{1/2}$Bi$_{1/2}$TiO$_{3−0.06}$BaTiO$_3$ has been considered with and without Zn doping and with quenching. It is compared to 0.79Na$_{1/2}$Bi$_{1/2}$TiO3−0.21K$_{1/2}$Bi$_{1/2}$TiO$_3$ with and without Mg doping. Finally, a contrast to soft Pb(Zr$_{1/2}$Ti$_{1/2}$)O$_3$ (PZT) flushes out the impact of domain wall motion on the piezoelectric coefficient and the electromechanical quality factor. Whereas a PZT-based reference material exhibits a linear increase in the piezoelectric constant with increasing remanent polarization, the NBT-based materials deviate from the linear trend, indicating a decrease in extrinsic contributions

Structural and microstructural description of relaxor-ferroelectric transition in quenched Na1/2Bi1/2TiO3–BaTiO3

Quenching lead-free Na1/2Bi1/2TiO3-based ceramics from sintering temperature is established to increase the depolarization temperature, Td and the lattice distortion. In situ synchrotron X-ray diffraction measurements were carried out on furnace cooled and quenched Na1/2Bi1/2TiO3 - BaTiO3 (NBT-BT) with 6 and 9 mol. % BT to discern the field-induced ferroelectric order. Phase fractions were determined from full pattern Rietveld refinements and utilized together with the change in unit cell volume to calculate volumetric strain resulting from phase transformations. NBT-6BT demonstrates a cubic symmetry in the furnace cooled state but quenching stabilizes the rhombohedral R3c phase and delays the formation of a field-induced, long range-ordered tetragonal phase, thereby shifting the onset of macroscopic strain to higher fields. A field-induced phase transition from a weakly distorted rhombohedral to tetragonal phase can be observed in furnace cooled NBT-9BT. However, this phase transition cannot be detected in quenched NBT-9BT, since the ferroelectric tetragonal P4mm phase is stabilized in the initial state. In contrast to the furnace cooled materials, both the quenched compositions exhibit overall negligible volumetric strain as a function of electric field. Furthermore, scanning electron micrographs of chemi- cally etched, poled and unpoled samples reveal an increased lamellar domain contrast in the quenched materials. All these findings strengthen the hypothesis of a stabilized ferroelectric order resulting in the absence of a field-induced phase transformation in quenched NBT-BT

Correlation between enhanced lattice distortion and volume fraction of polar nanoregions in quenched Na1/2Bi1/2TiO3–BaTiO3 ceramics

Quenching has been established as a viable method to increase the depolarization temperature in (100-x) Na1/2Bi1/2TiO3–xBaTiO3 (NBT–xBT). The proposed hypothesis of a stabilized ferroelectric order would entail changes in the polarized volume. To this end, air-quenched and furnace cooled samples of four compositions of NBT–xBT with x¼3, 6, 9, and 12 mol. % BT were studied. Upon quenching, all the compositions demonstrate an increase in the ferroelectric to relaxor transition temperature, TF-R, by 23–44°C and enhanced lattice distortion. Resonance frequency damping analysis was utilized to measure Young’s modulus in the temperature range of 25°C to 800°C and to estimate the volume fraction of polar nanoregions using a composite model. Quenching leads to an 8% decrease in Young’s modulus, but to an increase in the volume fraction of polar nanoregions by 12% at 300 °C for NBT-6BT. Transmission electron microscopy investigations of quenched NBT-6BT reveal a combination of lamellar domains and more homogenous areas with nanometersized domains. The existence of lamellar domains in quenched morphotropic phase boundary compositions together with enhanced lattice distortion and a decrease in dielectric frequency dispersion substantiate the premise of a stabilized ferroelectric order

Domain structure and phase evolution in quenched and furnace cooled lead-free Na1/2Bi1/2TiO3–BaTiO3 ceramics

Relaxor ferroelectric Na1/2Bi1/2TiO3-based materials have gained considerable attention as a potential lead-free alternative in recent years and can be tailored to exhibit giant strain or superior high power properties. Quenching (1-x)(Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-BT) ceramics in air from the sintering temperature is beneficial in enhancing the depolarization temperature and the lattice distortion. Here, a comparative study using transmission electron microscopy (TEM) and X-ray diffraction is presented for unpoled, furnace cooled and quenched NBT-BT (3, 6, 9 and 12 mol. % BT) ceramics describing the domain structure and phase assemblage. In contrast to the furnace cooled sample, an enhanced lamellar domain contrast is observed for the quenched morphotropic phase boundary composition with 6 mol. % BT. The phase fraction obtained using high resolution X-ray diffraction changes from a near pseudocubic structure with small distortions towards a more pronounced rhombohedral and tetragonal phase assemblage. On the NBT-rich side (3 mol. % BT), a second rhombohedral phase emerges in addition to the R3c symmetry, exhibiting a long-range lamellar domain structure. Further, quenched and subsequently poled NBT-6BT features an increased tetragonal fraction associated with a highly lamellar domain contrast. The quenching treatment stabilizes the ferroelectric order, evidenced from the development of a long-range ferroelectric domain structure, which rationalizes the enhanced depolarization temperature