Large electrostrain and structural evolution in (1-x)[0.94Bi0.5Na0.5TiO3-0.06BaTiO3]-xAgNbO3 ceramics

A lead-free system formulated as (1-x)[0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃]-xAgNbO₃ exhibits an electrostrain of 0.50% at 80 kV/cm and a maximum d₃₃^* of 721 pm/V at 60 kV/cm for x=0.3. The incorporation of AgNbO₃ shifts the relaxor-ferroelectric phase transformation temperature (T_F-R) to room temperature and lowers the energy barrier of the field-induced phase transformation. Furthermore, the in-situ electric field dependent high-energy synchrotron X-ray diffraction (SXRD) technique reveals that the sample x=0.03 transforms from dominant P4bm phase to a phase mixture of R3c + P4mm at 55 kV/cm during the electric field loading, and returns to initial dominant P4bm phase at 15 kV/cm during the unloading cycle of the electric field. Furthermore, it can be demonstrated that the electric field induced-phase transition in NBTBT-3AN occurs in the whole sample, rather than in the single direction of electric field. Therefore, the electrocstrain in NBTBT-3AN is more uniform, which would be beneficial to its actuator applications.     

Large electrostrain and high energy-storage of (1-x)[0.94(Bi0.5Na0.5) TiO3-0.06BaTiO3]-xBa(Sn0.70Nb0.24)O3 lead-free ceramics

(1-x)[0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃]-xBa(Sn_0.70Nb_0.24)O₃ (abbreviated as BNTBT-100xBSN) lead-free ceramics were fabricated with a relative density greater than 96 %, and the structure as well as performance were tested. BNTBT-100xBSN ceramics are pseudo-cubic perovskite structure, with dense surface morphology. Doping BSN can effectively reduce the dielectric loss of ceramics and increase the relaxation properties to a certain extent. The randomly distributed ferroelectric phase was replaced by polar nano regions, thereby improving the electro-strain and energy storage performance of the system. The largest electro-strain and the corresponding normalized strain (d₃₃*) reach ~ 0.43 % and 633 pm/V respectively in the BNTBT-1BSN ceramic. The largest effective energy storage density reaching ~ 1.28 J/cm³ was tested in BNTBT-2BSN. BNTBT-100xBSN ceramics provide a feasible idea for the systematic research of lead-free ferroelectrics and improvements in electro-strain and energy storage applications.     

Enhancement of pyroelectric properties of lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics by La doping

Lead-free 0.94NBT-0.06BT-xLa ceramics at x = 0.0–1.0 (%) were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. With increasing La³⁺ content pyroelectric coefficient (p) and figures of merits greatly increase; however, the depolarization temperature (T_d) decreases. p is 7.24 × 10⁻⁴C m⁻² °C⁻¹ at RT at x = 0.5% and 105.4 × 10⁻⁴C.m^−2 °C⁻¹ at T_d at x = 0.2%. F_i and F_v show improvements at RT from 1.12 (x = 0%) to 2.65 (x10 ⁻¹⁰ m v⁻¹) (x = 0.5%) and from 0.021 to 0.048 (m².C⁻¹) respectively. F_i and F_v show a huge increase to 37.6 × 10⁻¹⁰ m v⁻¹ and 0.56 m² C⁻¹ respectively at T_d at x = 0.2%. F_C shows values of 2.10, 2.89, and 2.98 (x10⁻⁹C cm⁻² °C⁻¹) at RT at 33, 100 and 1000 (Hz) respectively. Giant pyroelectric properties make NBT-0.06BT-xLa at x = 0.2% and 0.5% promising materials for many pyroelectric applications.     

0-3 type magnetoelectric 0.94Na0.5Bi0.5TiO3-0.06BaTiO3:CoFe2O4 composite ceramics with a deferred thermal depolarization

Developing Na_0.5Bi_0.5TiO₃-based magnetoelectric (ME) coupling composites with higher depolarization temperature is highly valuable for the environment-friendly smart electronic devices. We have developed a new kind of 0-3 type 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃:xCoFe₂O₄ (NBTBT:xCFO, x = 0, 0.1, 0.2, 0.3) composite ceramics with a deferred depolarization temperature, together with an additional strong ME coupling of 9.2 mV/cm·Oe for the NBTBT:0.2CFO. The basic structure, ferroelectric/ferromagnetic properties, and the depolarization temperature of the NBTBT:xCFO composite ceramics were investigated. It was found that an enhancement of depolarization temperature (>25 °C) was obtained in these 0-3 type composites relative to the pure NBTBT ones (115 °C vs 90 °C). The mechanism of the enhanced depolarization temperature of the composites is discussed. The present results demonstrate that NBTBT:xCFO composites have great potential for ME devices.     

The decrease of depolarization temperature and the improvement of pyroelectric properties by doping Ta in lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

Ta-doped lead-free 0.94NBT-0.06BT-xTa (x=0.0–1.0%) ceramics were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. The depolarization temperature (T_d) shifted to lower temperature with the increase of Ta content. The pyroelectric coefficient (p) of doped ceramics greatly enhanced compared with undoped material and reached a maximum of 7.14×10⁻⁴ C m⁻² °C⁻¹ at room temperature (RT) and 146.1×10⁻⁴ C m⁻² °C⁻¹ at T_d at x=0.2%. The figure of merits, F_i and F_v, also showed a great improvement from 1.12×10⁻¹⁰ m v⁻¹ and 0.021 m² C⁻¹ at x=0.0 to 2.55×10⁻¹⁰ m v⁻¹ and 0.033 m² C⁻¹ at x=0.2% at RT. Furthermore, F_i and F_v show the huge improvement to 52.2×10⁻¹⁰ m v⁻¹ and 0.48×10⁻¹⁰ m v⁻¹ respectively at T_d at x=0.2%. F_C shows a value between 2.26 and 2.42 ×10⁻⁹ C cm⁻² °C⁻¹ at RT at x=0.2%. The improved pyroelectric properties make NBT-0.06BT-0.002Ta ceramics a promising infrared detector material.     

Phase/domain structure and enhanced thermal stable ferro-/pyroelectric properties of (1-x)0.94Na0.48 Bi0.44TiO3-0.06BaTiO3:xZnO ceramics

Composite ceramics based on non-stoichiometric 0.94Na_0.48Bi_0.44TiO₃ (NBT-BT) and ZnO, (1-x)NBT-BT:xZnO with x = 0 ∼ 0.3 are prepared and investigated. The phase structure changes from tetragonal-dominated for x = 0 to rhombohedral-dominated for x > 0. The domain structure simultaneously evolves from nanodomain to macrodomain. At room temperature, the x = 0 ceramic has a pinched polarisation-electric field (P-E) loop, whereas x > 0 ceramics demonstrate square-shaped P-E loops with a maximum P_r of 30.6 μC/cm² of x = 0.2. Moreover, ceramics with x = 0.1 and 0.2 have enhanced pyroelectric coefficients in a wide temperature range with full width at half maximums of 31 °C and 15.5 °C and peak values of 29.2 × 10⁻⁴ and 100.7 × 10⁻⁴ C/(m²·K), respectively. The results are explained based on the stress-induced room temperature relaxor-ferroelectric transition. This work affirms that forming composites is an effective way to tune the electrical properties of NBT-based materials.     

Tuning the electrocaloric effect in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by relaxor phase blending

The pseudo-first-order phase transition in 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ ceramics leads to a sharp increase in temperature change (ΔT) in the vicinity of the ferroelectric-to-relaxor transition temperature T_FR (~100 °C) [Appl. Phys. Lett. 110 (2017) 182904]. In this study, we add the 0.78Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.16(Sr_0.7Bi_0.2)TiO₃ relaxor phase to the 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ ferroelectric matrix to tune its electrocaloric effect. The results show that the addition of the relaxor phase plays a vital role in phase and local-structure evolution. A transition occurs between the ferroelectric and ergodic relaxor phases when the mass fraction of the latter increases to 30% (x = 0.3), as verified by X-ray diffraction analysis, Raman spectroscopy, and polarization-electric field (P-E) hysteresis loops. Furthermore, addition of the relaxor phase reduces the T_FR from 76 °C at x = 0.1–55 °C at x = 0.2; however, this transition disappears at x = 0.3 and 0.4 composite. In-situ piezo-force microscopy (PFM) images illustrate that domains can be written into x = 0.1 and 0.2 ceramics with a valley in the piezoresponse curves. Increasing the temperature agitates the domain arrangement and decreases the contrast for PFM images; this indicates a gradual phase transition in the composite. The temperature corresponding to maximum ΔT exhibits a downward shift (0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2), while the temperature-ΔT curves are flat when x = 0.3 and 0.4. Moreover, the maximum ΔT shows a decrease with an increase in the relaxor phase content; this is believed to be related to a decrease in the latent heat due to a pseudo-first-order to second-order transition. Thus, we suggest that the incorporation of a relaxor phase into ferroelectric matrices is an effective technique to tune their electrocaloric effect and improve the thermal stability of ceramic composites.     

(0.94-y)Bi_(0.5)Na_(0.5)TiO_3-0.06BaTiO_3-yBiCoO_3陶瓷的制备与性能

为发展绿色无毒的PZT(lead zirconate titanate,锆钛酸铅)压电陶瓷替代压电材料,钛酸铋钠因其剩余极化强和压电性能好等特点被广泛研究。为进一步提高其压电性能,我们利用传统固相合成工艺制备了(0.94-y)Na_(0.5)Bi_(0.5)Ti O3-0.06Ba Ti O3-y Bi Co O3无铅压电陶瓷。探究了在Ba Ti O3定量的情况下,Bi Co O3对陶瓷的热学性质、结构、压电性能的影响。实验结果表明,微量添加Bi Co O3对BNBT6的结构基本没有影响,所制备的陶瓷仍均为单一的钙钛矿结构;随着Bi Co O3含量的增加,剩余极化强度Pr和矫顽场Ec都是先降低后增加;通过对陶瓷样品的压电性能分析,可以看出Bi Co O3的掺杂量为0.01时,陶瓷样品的压电常数d33取得最大值171 p C/N。     

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(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ (BNT-BT) piezoelectric ceramics. At room temperature, a large strain of 0.6% (at 70 kV/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 T_F-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.     

BaTiO3 nanowires-induced phase transition and thermally stable strain in (Bi0.5Na0.5)TiO3 piezoelectric ceramics

Environment-friendly lead-free piezoceramics with high strain response and extremely excellent stability in a wide operating temperature range are critically important in practical actuator applications. Here, we develop a new strategy to tune the electrostrictive strain behavior in Bi_0.5Na_0.5TiO₃ (BNT)-based ceramics via using high aspect ratio BaTiO₃ nanowires (BT NWs) as a modifier. The addition of BT NWs generates a crossover from a typical ferroelectric (BT conventional spherical particles) to a complete ergodic relaxor (ER) phase at ambient temperature, accompanied by a large electrostrictive strain of ∼0.17% with d₃₃*(S_max/E_max) = 284 pm/V. Such a high electrostrictive strain is extremely thermally stable with only <7% fluctuation from 27 °C to 120 °C. In addition, the BT NWs-modified ceramics also exhibit acceptable fatigue endurance (<30% up to 10⁵ cycles) and frequency dependence (<20% at 10Hz–100Hz). These achieved exceptional performances can be ascribed to the BT NWs-driven complete ER phase at room temperature. The findings of this study can inspire enhanced interest in nanowires as a viable modifier to BNT-based materials due to promising potential for practical actuator applications in a wide temperature range.     

Antiferroelectricity in tantalum doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06(Ti_1−xTa_x)O₃ (x=0.00–0.04) lead-free polycrystalline ceramics were synthesized using the solid state reaction route, and their crystal structures and electrical properties were systematically studied. With the introduction of Ta substitution, the relaxor antiferroelectric phase with tetragonal P4bm symmetry is stabilized. The representative double polarization hysteresis loops and sprout shaped strain curves for antiferroelectric ceramics are observed at higher Ta contents with x=0.01–0.02 at room temperature. x=0.01 shows the largest strain of 3.81‰ under 60 kV/cm, indicating a good candidate for actuator applications. The polarization and strain hysteresis loops are also evaluated to verify the temperature-induced normal ferroelectric phase to relaxor antiferroelectric phase transition at temperature up to 120 °C. The energy storage density and efficiency at various temperatures are calculated and analyzed in the compositions of x=0.00–0.02. The results indicate that the energy storage density becomes more temperature independent with the increase of Ta concentration, which are promising for applications in high-temperature capacitors.     

Polar domain structural evolution under electric field and temperature in the (Bi0.5Na0.5)TiO3-0.06BaTiO3 piezoceramics

Lead-free bismuth sodium titanate and related compounds are of great interest as promising candidates for piezoelectric applications. However, the full understanding of this family of materials is still a challenge partly because of their structural complexity and different behaviors with or without the application of an external electric field. Here, piezoresponse force microscopy is used to gain insight into the mesoscopic-scale domain structure of the morphotropic phase boundary (MPB) composition of (1-x)Bi_0.5Na_0.5TiO₃-xBaTiO₃ solid solution at x = 0.06 (abbreviated as BNT-6BT). The evolution of the domains with the changes of the electric field and temperature has been thoroughly examined in conjunction with the crystal structure analysis and dielectric studies. It is found that ferroelectric domains with size of hundreds of nanometers are embedded in a relaxor state without visible domains on a mesoscopic scale, which are considered to contribute to the tetragonal and cubic phases in the material, respectively. Temperature-independent domain configuration is observed in the unpoled sample from room temperature to 200°C. While, temperature-dependent domain configuration is observed in the poled sample. The homogenously poled state breaks into the mixed domain configuration containing polydomain structure and invisible state around the so-called depoling temperature. The structural changes on different length scales are also discussed. This work provides an in-depth understanding of the structural and domain changes under an electric field and the temperature-dependent domain evolution in both unpoled and poled states in the BNT-BT solid solution of the MPB composition.     

Giant electric field-induced strain and structure evolution of NaTaO3-modified 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 Pb-free ceramics

To acquire giant electric field-induced strain in non-Pb materials is attracting a great deal of attention in the past decade. In the current investigation, the crystal and domain structures as well as the electrical performances of (1-x) (0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃)-xNaTaO₃ (BNBT-xNT) specimens were systematically studied to achieve enhanced strain. The introduction of NT makes the phase structure transit from rhombohedral-tetragonal to pseudo-cubic structure. The original domain structure of BNBT is destroyed, and the disorder degree of the local structure increases. Simultaneously, the remnant polarization, coercive field, and piezoelectric coefficients were significantly decreased. The transition from ferroelectric to ergodic relaxation can be effectively modified, thus lowering the transition zone to room temperature. Finally, the BNBT-3NT ceramics achieve marked strain coefficients at room temperature, with a maximum strain of 0.394% under 65 kV/cm and a d₃₃* of 606 pm/V.     

Enhanced temperature stable dielectric properties and energy-storage density of BaSnO3-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

A series of (1-x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-xBaSnO₃ (BNBT-100xBSN, x = 0–20) lead-free ceramics were synthesized using a conventional high-temperature solid-state reaction route. The effects of BaSnO₃ on the dielectric, ferroelectric and energy-storage performance of BNBT-BSN were systematically investigated. Temperature dependent permittivity curves indicated the obviously enhanced relaxor ferroelectric property. The introduction of BaSnO₃ reduced the temperature corresponding to the first dielectric anomaly, which facilitated the dielectric temperature stability. △ε'/ε'_150°C varied no more than 15% within the temperature range of up to 338 °C (45–383 °C) for BNBT-15BSN. A slimed P-E loop was obtained with the remnant polarization of 0.4 μC/cm² for BNBT-15BSN. Moreover, the breakdown field intensity of BNBT-BSN increased effectively from 80 kV/cm to 115 kV/cm. Therefore, an optimum energy-storage performance was obtained in BNBT-15BSN with the energy-storage density of 1.2 J/cm³ whose energy-storage efficiency reached 86.7%. Furthermore, the possible contributions of defect and vacancy to relaxation and conductance mechanism were discussed by studying the impedance and electric modulus. The results above indicated the BNBT-100xBSN be a promising lead-free candidate for energy-storage capacitors.     

Large field-induced strain with enhanced temperature-stable dielectric properties of AgNbO3-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(1-x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-xAgNbO₃ lead-free piezoelectric ceramics (abbreviated as BNBT-100xAN) were prepared using the conventional solid-state sintering method. The effects of the introduction of AgNbO₃ (AN) dopants for the dielectric and piezoelectric performances of BNBT-100xAN ceramics were systematically studied. The XRD patterns and Raman spectra demonstrated that AN as a modifier was successfully diffused into the BNBT-100xAN lattice and revealed a pseudo-cubic symmetry structure. All samples exhibited a dense surface morphology accompanied by the uniform distribution of elements. A large bipolar strain of ~0.501% and unipolar strain of ~0.481% corresponding to the normalized strain d₃₃* of ~740 p.m./V were achieved for BNBT-1AN ceramic at 65 kV/cm field. The BNBT-4AN ceramic exhibited an excellent temperature-stable permittivity with the range from 59 to 380 °C and its dielectric loss was less than 0.02 between 97 °C and 329 °C. These results revealed that BNBT-100xAN ceramics were more hopeful candidates for actuators, strain sensors, and high-temperature capacitors.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

Energy storage property of (Pb0.97La0.02)(Zr0.5Sn0.4Ti0.1)O3-(Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics: Effects of antiferroelectric-relaxor transition and improved breakdown strength

(1-x)(Pb_0.97La_0.02)(Zr_0.5Sn_0.4Ti_0.1)O₃-x(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (x = 0 ∼ 0.4) ceramics have been prepared and investigated. The ceramics consist of perovskite solid solution matrix and precipitated, isolated SnO₂ particle, resulting in 0–3 type composite structure. With increasing x value, the room temperature crystal structure of perovskite solid solution transforms from tetragonal to pseudocubic, therefore, the electrical property evaluates form robust antiferroelectric at x = 0, metastable antiferroelectric at x = 0.1, and then relaxor ferroelectric at x > 0.1. Moreover, the breakdown strength is enhanced due to the composite structure and reaches maximum value of 190 kV/cm at x = 0.2. Both the phase transition and enhanced breakdown strength are helpful to improve energy storage property, the x = 0.2 ceramic shows largest recoverable energy density w_rec of 1.84 J/cm³, discharge efficiency $\eta$ of 86.6 %. Especially, both w_rec and $\eta$ illustrates significantly improved thermal stability within 25−125 °C.     

Polarization behavior of` lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin films with enhanced ferroelectric properties

(Bi_0.5Na_0.5)TiO₃ based ferroelectric lead-free thin films have great potential for modern micro-devices. However, the multicomponent feature and volatile nature of Bi/Na makes the achievement of high quality films challenging. In this work, the morphotropic phase boundary composition, 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ thin films were successfully prepared by CSD method. Dense films with low dielectric loss and low leakage current density were obtained. A well-defined polarization hysteresis loop with a high remnant polarization was observed in the thin films. Moreover, the polarization behavior of the film at original state, under electric field and upon heating was investigated by PFM. A self-polarization and asymmetric domain switching behavior were observed. High temperature induced depolarization and the self-polarization recovered upon cooling. The thin films with good quality show a promising potential for the application in electrical devices, and the in-depth investigation of the polarization behavior improves the understanding of ferroelectric and piezoelectric properties of thin films.