Ferroelectric-relaxor Transition Temperature Research Articles

Polarization extension yielding ultrahigh piezoelectric response in xPb(Nb2/3Ni1/3)O3-(1-x)Pb(Zr0.3Ti0.7)O3 ferroelectrics ceramics

The heightened piezoelectric performance observed in most explored perovskite systems is typically attributed to the electric-field-induced phase transition near the morphotropic phase boundary (MPB) or polymorphic phase boundary (PPB). This study, however, unveils a distinct piezoelectric enhancement mechanism in the xPb(Nb2/3Ni1/3)O3-(1–x)Pb(Zr0.3Ti0.7)O3 (PNN-PZT) system, diverging from the MPB/PPB-centered piezoelectric systems. Notably, the composition with x = 0.55, positioned close to the tetragonal-pseudocubic (T-PC) phase boundary, achieves an unprecedented piezoelectric coefficient (d33) of 1264 pC/N, while retaining a tetragonal local structure. Importantly, on a local scale, electric fields do not incite phase transitions, suggesting that the exceptional piezoelectric performance in PNN-PZT stems from polarization extension near the T-PC boundary. Distinct from other mechanisms, the relative permittivity significantly increases post-poling due to this particular enhancement process. The dielectric behavior in poled specimens does not exhibit a conspicuous change at the ferroelectric-relaxor transition temperature. However, both the relative permittivity and planar electromechanical coupling coefficient experience a sharp rise in the temperature range of −25 °C to 25 °C. This investigation underscores the pivotal role of designing compositionally-driven T-PC phase boundaries, presenting a promising avenue for enhancing piezoelectric properties in ferroelectric ceramics.

Effects of Na Excess on the Crystal Structure Ferroelectric and Piezoelectric Properties of (Bi0.487Na0.427K0.06Ba0.026)TiO3 Lead-Free Piezoceramics

In this research, (Bi0.487Na0.427K0.06Ba0.026)TiO3 ceramics were prepared by conventional solid-state mixed-oxide method to incorporate Na excess according to the chemical formula (Bi0.487Na0.427+ x K0.06Ba0.026)TiO3, where x = 0.0, 0.005, 0.010, 0.015 and 0.020. The crystal structure, dielectric, ferroelectric and piezoelectric properties were systematically investigated with respect to the amount of Na excess. X-ray diffraction data identified pure perovskite structure for all compositions. The pseudocubic structure was observed for x = 0.0 − 0.005 before it transformed to rhombohedral structure at x ≥ 0.015. The grain size tended to be larger with increasing Na excess amount. Increasing x also led to a substantial increase in ferroelectric-relaxor transition temperature (T F-R) from 80 °C (x = 0) to 110 °C (x = 0.020). With increasing x from 0 to 0.020, the standard polarization-electric field (P-E) hysteresis measurements revealed a reduction in remanent polarization (P r) together with an increase in coercive field (E c), which was further confirmed by the remanent P-E hysteresis measurements. In addition, the piezoelectric constant (d 33) was also found to steadily decrease as x increased. The observation of increased T F-R and E c along with a decrease in d 33 and P r indicated a stabilization of ferroelectric order induced by Na excess. This research highlights the importance of sufficient control over A-site stoichiometry and how it affects the ferroelectric and piezoelectric properties of BNT-based ceramics.

Simultaneously enhanced piezoelectric response and depolarization temperature in BNT-7BT ceramics with simple composition via optimizing sintering temperature

For (Bi0.5Na0.5)TiO3-based ceramics, one of the obstacles limiting their actual application is the incompatibility of a large piezoelectric response d33 with a low depolarization temperature Td. In this work, a high d33 of 186 pC/N and a Td of 133 °C are achieved simultaneously in unmodified 0.93(Bi0.5Na0.5) TiO3–0.07BaTiO3 (BNT-7BT) ceramics only via optimizing sintering temperature. Structural and microstructural analyses show that grain size and relative content of P4bm and R3c phases in BNT-7BT ceramics can be regulated by tuning the sintering temperature (Tsinter = 1100 °C, 1120 °C, 1140 °C, 1160 °C, and 1180 °C). Thus, high sintering temperature is not only able to increase the grain size to promote the domains switching for higher the piezoelectric response, but also to enhance the ratio of R3c/P4bm phase so as to improve ferroelectric-relaxor transition temperature that defer the depolarization. Hence, the present work provides a simple and effective method for optimizing the piezoelectric properties of (Bi0.5Na0.5)TiO3-based ceramics.

Enhanced temperature stability of ferroelectric properties in lead-free (1−x)(Bi0.5Na0.4K0.1)TiO3-x(Ba0.5Sr0.5)TiO3 solid solutions

With its complicated composition architecture, (Bi 0.5 Na 0.5 )TiO 3 (BNT)-based relaxor ferroelectrics exhibit distinctive dielectric relaxation phenomena and attractive characteristics. The structural, dielectric, and ferroelectric characteristics of (1– x )(Bi 0.5 Na 0.4 K 0.1 )TiO 3 - x (Ba 0.5 Sr 0.5 )TiO 3 (BNKT- x BST) solid solutions with x = 0–0.4 were thoroughly examined in this study. The x = 0.1 and 0.2 versions exhibited unexpected ferroelectricity enhancement, and the depolarization temperatures and ferroelectric-relaxor transition temperature were both increased compared to that of the BNKT undoped sample. This enhancement is due to the creation of a tetragonal ferroelectric phase after the introduction of a moderate BST member, according to room temperature and temperature-dependent x-ray diffraction data. The characterization of ferroelectric properties indicated that ferroelectricity was enhanced by increasing the rapidly declining temperatures for the remnant polarization and coercive field, compared to those of the BNKT undoped sample. This discovery contributes to a better understanding of the relationship between the structure and properties of complex relaxor ferroelectrics. A phase diagram of the BNKT- x BST solid solution is also provided, which can be used to guide the search for specific lead-free functional materials for the capacitors, actuators, and transducers based on this eco-friendly system.

Synergy of ferroelectric-relaxor transition and multiphase coexistence induced high electrocaloric effect with wide temperature span in Pb0.7Ba0.3ZrO3-based system

In this work, (1-x)Pb0.7Ba0.3ZrO3–xBi(Mg0.5Ti0.5)O3 (PBZ–xBMT) compositions was designed with an integration of ferroelectric–relaxor transition and multiphase coexistence to synergistically optimize electrocaloric effect (ECE) and temperature stability. Rietveld refinements reveal a phase structure change with sequence of R3cH → R3cH + Pm3¯m → Pm3¯m as BMT content increases, whilst relaxation property also increases with downshift in ferroelectric–relaxor transition temperature. Structural evolution is also verified by in-situ Raman spectra and ferroelectric measurements. Interestingly, a large ECE (ΔT ∼ 0.86 K) with a broad temperature span (ΔTspan ∼ 58 K) was successfully obtained in x=0.01 composition, which is believed to be contributed by diffuse phase transition character with multiphase coexistence. This work provides a guideline to exploring ferroelectric materials with high ECE/wide operation span and the reported PBZ–based ceramics with a large ΔT and ΔTspan is a promising candidate in application of solid–state cooling devices.

Low driving field-induced large strain in ternary lead-free Na0.5Bi0.5TiO3-SrTiO3-ABO3 piezoceramics

In this letter, three ternary lead-free Na0.5Bi0.5TiO3-SrTiO3-ABO3 piezoceramics (ABO3 = LiNbO3, BaZrO3, and LaFeO3) were studied. The addition of ABO3 into Na0.5Bi0.5TiO3-SrTiO3 ceramic induces a ferroelectric tetragonal-to-ergodic relaxor pseudocubic phase transition, resulting in the significant disruption of ferroelectric order with the shift of the ferroelectric-relaxor transition temperature down to below room temperature. The conventional furnace sintering and microwave sintering methods are conducted in this study to improve the electric field-induced strain properties of BNST-LN piezoceramic. Interestingly, the Na0.5Bi0.5TiO3-SrTiO3-LiNbO3 sample sintered via microwave sintering method shows an ultrahigh normalized strain of 995 pm/V at an ultralow driving field of 2 kV/mm, suggesting a potential candidate for lead-free piezoactuator applications.

Optimization of Ferroelectric Ordering and Thermal Stability in Na1/2Bi1/2TiO3-Based Lead-Free Single Crystal through Defect Engineering.

Environmentally friendly lead-free piezoelectric materials have been attracting significant attention in recent years. Na1/2Bi1/2TiO3-based relaxor ferroelectrics have found acceptance for application in promising lead-free transducers in high-power ultrasonic devices. However, their low thermal stability, i.e., their relatively low ferroelectric-relaxor transition temperature (TF-R), hinders their practical application. Herein, a thermal-quenching approach is applied on a Na1/2Bi1/2TiO3 (NBT)-based single crystal, which yields a large increase in TF-R and dramatic enhancement of its ferroelectric ordering, leading to excellent thermal stability of its dielectric, ferroelectric, and piezoelectric properties. This behavior is mainly attributed to quenching-induced domain evolution as well as its octahedral tilt, which is linked to the increased oxygen vacancies. The substitution of long-range ordered ferroelectric domains for short-range polar nanodomains contributes to its increased coherence length and, consequently, enhancement of TF-R. This work provides an approach to the optimization of the ferroelectric ordering and thermal stability of NBT as well as an in-depth understanding of the quenching effect on the local structure, which could be applied to other relaxor-based ferroelectrics for optimization of their macroscopic properties.

Enhanced strains of Nb-doped BNKT-4ST piezoelectric ceramics via phase boundary and domain design

A strategy that constructs the morphotropic phase boundary and manipulates the domain structure has been used to design the component of 0.96[Bi0.5(Na0.84K0.16)0.5Ti(1-x)NbxO3]-0.04SrTiO3 (BNKT-4ST-100xNb) to enhance the strain properties for actuator application. Non-equivalent Nb5+ donor doping modulates the phase transition from the mixture of rhombohedral and tetragonal phases to the pseudocubic phase and results in the coexistence of multiple phases. Moreover, the high-resolution TEM confirms the existence of polar nano regions that contribute to the macroscopic relaxor behaviour. The size of the domains is reduced with increasing Nb5+, resulting in an enhanced relaxor behaviour. The ferroelectric-relaxor transition temperature decreases from 85 to below 30 °C, implying a non-ergodic to ergodic relaxor transition. An improved strain of 0.56% and a giant normalized strain of 1120 pm/V were achieved for BNKT-4ST-1.5Nb, which were attributed to the unique domain structure in which nanodomains are embedded in an undistorted cubic matrix. Ferroelectric, antiferroelectric, and relaxor phases coexist. As the electric field is large enough, a reversible phase transition occurs. Furthermore, good temperature stability was obtained due to the stability of the nanodomains, and no degradation in strains was observed even after 104 cycles, which may originate from the reversible phase transition and dynamic domain wall. The results show that this design strategy offers a reference way to improve the strain behaviour and that BNKT-4ST-100xNb ceramics could be a potential material for high-displacement actuator applications.

Low driving field-induced large strain in MnO2-modified 0.76Bi1/2Na1/2TiO3-0.24SrTiO3 lead-free piezoceramics

MnO2-modified 0.76Bi1/2Na1/2TiO3-0.24SrTiO3 (BNT-24ST) lead-free piezoceramics were synthesized by a conventional solid-state method. The introduction of MnO2 influences the phase structure and increases the grain size. Increasing the MnO2 content shifts the ferroelectric-relaxor transition temperature to below room temperature. This leads to a transition from ferroelectric behavior to ergodic-relaxor behavior. A large electrostrain of 0.18% and a normalized strain (Smax/Emax) of 902 pm/V with an ultra-low driving field of 2 kV/mm are achieved for the 0.25 wt% MnO2 doped BNT-24ST. A temperature-insensitive Smax/Emax of above 610 pm/V from room temperature to 90 °C is achieved at 4 kV/mm. This is attributed to the modification by MnO2 and the low poling field. Excellent mechanical performance of Young’s modulus E~127.4 GPa and fracture toughness KIC~1.38 MPa.m1/2 are also obtained, which are better than those reported for PZT-based ceramics. The results indicate the BNT-24ST with MnO2 ceramic is a promising candidate for piezoactuator applications.

Enhanced relaxor behavior and thermal- and frequency-insensitive strain of (Na0.5Bi0.5)0.93Ba0.07Ti1 − x(Mn1/3Nb2/3)xO3 ceramics

(Na0.5Bi0.5)0.93Ba0.07Ti1 − x(Mn1/3Nb2/3)xO3 ceramics (x = 0–0.02) were prepared, and their structure and property were investigated systematically. (Mn1/3Nb2/3)4+ doping shifts ferroelectric–relaxor transition temperature (TF–R) toward low temperature. Thus, ferroelectric order is suppressed and relaxor behavior is enhanced monotonously, as further evidenced by composition-dependent electrical properties and domain evolution from labyrinthian to nanodomain. As a result, the x = 0.010 ceramic yields a large room temperature field-induced strain of 0.37%, especially the strain is temperature- and frequency-insensitive with a variety of 4.3% and 4.1% in the ranges of 25–125 °C and 0.5–10 Hz, respectively. This work suggests that the low content (Mn1/3Nb2/3)4+ complex cation doping is an alternative method to effectively tune the electrical properties of Na0.5Bi0.5TiO3-based ferroelectrics.

Highly enhanced thermal stability in quenched Na0.5Bi0.5TiO3-based lead-free piezoceramics

Unquenched and quenched ceramics of 0.85Na0.5Bi0.5TiO3-0.11K0.5Bi0.5TiO3-0.04BaTiO3 have been prepared, and their crystal structure, temperature-dependent ferro-/piezoelectric properties and domain structure have been comparatively investigated. It is shown that quenching process can significantly improve the ferroelectric-relaxor transition temperature (TF-R), which is 130 °C for unquenched ceramics and 198 °C for quenched one. As the result, the thermal stability of ferro-/piezoelectric properties is highly enhanced. These observations are mainly attributed to the quenching induced stable rhombohedral ferroelectric phase and the defect altered domain evolution. This work may deepen the understanding of the effect of quenching on crystal structure, domain structure and their contributions to thermal stability of NBT-based ceramics.

Bi0.48(Na0.84K0.16)0.48Sr0.04(Ti1-xTax)O3 lead-free ceramics with enhanced electric field-induced strain

A series of Bi0.48(Na0.84K0.16)0.48Sr0.04(Ti1-xTax)O3 (abbreviated as NKS-100xTa) lead-free piezoelectric ceramics were fabricated using a traditional solid-state method. The structure and electrical properties of NKS-100xTa were investigated in detail. The composition for x = 0.01 presented an enhanced electric field induced bipolar strain of ∼0.53%, as well as the giant unipolar strain of ∼0.478% corresponding to Smax/Emax ∼682 p.m./V at 70 kV/cm. It is found that the ferroelectric-relaxor transition temperature (TF-R) shifted down to ambient temperature and the remnant polarization (Pr) decreased with the Ta doping. It destroyed the long-range ferroelectric ordering and increased the relaxor characteristic. Therefore, the origin of giant strain was ascribed to the transformation from the ferroelectric phase to relaxor phase under the applied electric field. In addition, the well fatigue properties within 104 cycles indicated that the prepared ceramics offered extendable availability for sensors and actuators.

Resolution of ambiguity between the depolarization and ferroelectric–relaxor transition temperature through dielectric studies in lead-free perovskite K0.5Bi0.5TiO3

Lead-free relaxor K0.5Bi0.5TiO3 (KBT) has been selected to measure its depolarization temperature (Td) and to establish the difference between the Td and the normal ferroelectric to relaxor transition temperature (TF-R), which is still in a debatable stage for the relaxor ferroelectric system. These parameters are determined separately from the electrically poled specimen through the simple dielectric measurement instead of the pyroelectric measurement or other sophisticated techniques. Albeit, the electric field induces the normal ferroelectric behavior in the nonergodic relaxor state, the relaxor nature appears again through the heating above the freezing temperature. The difference between the frequencies of resonance and anti-resonance peaks in the real part of permittivity remains constant up tõ240 °C, and it reduces to almost zero at ∼285 °C, which has been considered as the Td of KBT. At the temperature higher than Td, an anomaly is noticed in the temperature dependent permittivity as well as in the dielectric loss graph of KBT; this temperature indicates the emergence of the relaxor nature from the normal ferroelectric (TF-R). The Td of the KBT is even higher than some of the lead-free ferroelectrics, which signifies its better suitability for the piezoelectric application. This easy method can be adopted undoubtedly to confirm the Td of the ferroelectric specimen, even for the material of higher Td (above 300 °C).

The evolution of phase structure, dielectric, strain, and energy storage density of complex-ions (Sr1/3Nb2/3)4+ doped 0.82Bi0.5Na0.5TiO3-0.18Bi0.5K0.5TiO3 ceramics

Lead-free Bi0.5(Na0.82K0.18)0.5Ti1-x(Sr1/3Nb2/3)xO3 (abbreviated as BNKT-xSN, x = 0.02–0.045) ceramics were fabricated via the conventional ceramic process, and the (Sr1/3Nb2/3)4+ complex-ions were used to modify the phase transition and multifarious electrical responses of BNKT-xSN ceramics. The SN complex-ions absolutely incorporate into the lattice of BNKT matrix to form perovskite structure. The grain morphologies and size are almost unaffected by SN complex-ions. The phase transition concerning ferroelectric type transforming into ergodic relaxor happens with increasing SN content, and the corresponding critical point is x = 0.035. The phase transformation process results in the improvement of energy storage density (W = 0.754 J/cm3) at 80 kV/cm and a high bipolar strain (S = 0.25%) with small hysteresis. The dielectric constant at Tm peaks gradually decreases with increasing SN content, and the ferroelectric-relaxor transition temperature (TF-R) is depressed to room temperature. The evolution behaviors might facilitate our cognition about the mechanism between phase structure and multiple electrical properties of BNT-based ceramics.

Electric Field-Induced Large Strain in Ni/Sb-co Doped (Bi0.5Na0.5) TiO3-Based Lead-Free Ceramics

Lead-free piezoelectric ceramics (Bi0.5Na0.5)0.935Ba0.065Ti1−x (Ni0.5Sb0.5) x O3 (BNBT6.5-xNS) have been fabricated using conventional solid sintering technique. The effect of (Ni, Sb) doping on the phase structure and electrical properties of BNBT6.5 ceramics were systematically investigated. Results show that the addition of (Ni, Sb) destroyed the ferroelectric long-range order of BNBT6.5 and shifted the ferroelectric–relaxor transition temperature (T F–R) down to room temperature. Thus, this process induced an ergodic relaxor phase at zero field in samples with x = 0.005. Under the electric field, the ergodic relaxor phase could reversibly transform to ferroelectric phase, which promotes the strain response with peak value of 0.38% (at 80 kV/cm, corresponding to d 33 * = 479 pm/V) at x = 0.005. Temperature-dependent measurements of both polarization and strain confirmed that the large strain originated from a reversible field-induced ergodic relaxor to ferroelectric phase transformation. The proposed material exhibits potential for nonlinear actuators.

Synthesis and piezoelectric properties of (1 − x)Bi0.5(Na0.8K0.2)0.5TiO3–xSr2ZrTiO6 ceramics

The effects of Sr2ZrTiO6 (SZT) addition on the piezoelectric properties of (1 − x)Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT)–xSZT ceramics were characterized in this study. The X-ray powder diffraction (XRPD) profiles and Raman spectra of the ceramics in the composition range of 0–0.02 implies the presence of morphotropic phase boundary (MPB) which consists of the rhombohedral and tetragonal phases. Moreover, the temperature dependence of dielectric loss indicated a presence of the ferroelectric–relaxor transition temperature (TF–R) of around 75 °C for x = 0.005 and the temperature dependence shifted to a lower temperature at x = 0.01. The temperature dependence of the P–E hysteresis loop of the ceramics at the compositions of x = 0.005–0.02 showed pinched hysteresis loops above TF–R. Regarding the piezoelectric constant (d33), it was increased by SZT addition in the MPB region (x = 0–0.01) and the highest d33 of 202 pC/N was obtained at the composition of x = 0.0025. The S–E unipolar loop was also evaluated, the strain of the ceramic increased up to x = 0.02; and the highest pm/V was obtained at the composition of x = 0.02.

Electric field–induced large strain of (Bi1/2Na1/2)0.935Ba0.065TiO3–CaYAlO4 lead–free ceramics

Lead–free piezoelectric ceramics (1−x)(Bi1/2Na1/2)0.935Ba0.065TiO3−xCaYAlO4 (BNBT6.5−xCYAO) were prepared using a conventional solid sintering technique. The addition of CYAO destroyed the ferroelectric long–range order with the shift of the ferroelectric–relaxor transition temperature TF–R down to room temperature and induced the appearance of ergodic relaxor phase. Thus, it leads to a giant field–induced strain with a peak value of 0.44% at x=0.012.

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

In this study, the Bi-nonstoichiometric 0.99Bix(Na0.8K0.2)0.5TiO3-0.01SrTiO3 (BNKST) ceramics with x=0.5–0.535mol (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 (TFR) can be effectively tuned to around room temperature by Bi nonstoichiometry, giving rise to an enhanced room-temperature strain property. The positive strain Spos and dynamic piezoelectric constant d33* of Bi52.5 critical composition reach 0.33% and 440 pm/V, respectively at 6kV/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.

Electrocaloric effect and pyroelectric energy harvesting of (0.94 − x)Na0.5Bi0.5TiO3-0.06BaTiO3-xSrTiO3 ceramics

Ceramics with the composition (0.94−x)Na0.5Bi0.5TiO3–0.06BaTiO3–xSrTiO3 (NBBSTx) where x=0.10, 0.15, 0.20, and 0.25 were synthesized by a conventional solid-state reaction method to investigate their electrocaloric effect (ECE) and pyroelectric energy harvesting (PEH) properties. The ferroelectric, dielectric, and pyroelectric properties of the prepared ceramics were measured and discussed. It is found that the strontium titanate (ST) content and bias field greatly affect the ferroelectric–relaxor transition. Increasing ST content lowers the depolarization temperature of the ceramics, and both the ECE and PEH behavior of the ceramics strongly depend on their ST content because of the composition-induced decrease of the ferroelectric–relaxor transition temperature. The present investigation demonstrates that the ECE and PEH properties of NBBSTx ceramics can be tuned by introducing ST. Furthermore, a high PEH density of 425kJ/m3 is obtained for NBBST0.20, which is much higher than those of conventional Pb-based ferroelectrics.

Giant electric field-induced strain at room temperature in LiNbO3-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3

We report experimental investigation on the ferroelectricity and electric field-induced strain response in LiNbO3-doped 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNT-BT) piezoelectric ceramics. At room temperature, a large strain of 0.6% (at 70kV/cm) is achieved in the 2.5%-LiNbO3-doped BNT-BT, higher than that of commercially-utilized Pb(Zr,Ti)O3. The corresponding piezoelectric coefficient d*33 reaches 857 pm/V, which is high among these of BNT-based ceramics at room temperature. Further study indicates that the superior piezoelectric properties are realized at the ferroelectric-relaxor transition temperature TF-R, which is pushed to room temperature with 2.5% LiNbO3 doping. This indicates that large electromechanical response can be induced via delicate mixing of the ferroelectric rhombohedral phase and the polar nanoregions (PNRs) relaxor-ferroelectric tetragonal phase.