Large strain response in (Mn,Sb)–modified (Bi0.5Na0.5)0.935Ba0.065TiO3 lead–free piezoelectric ceramics

Lead–free piezoelectric ceramics (Bi_0.5Na_0.5)_0.935Ba_0.065Ti_1–x(Mn_0.5Sb_0.5)_xO₃ (BNBT6.5–xMS, x=0.005, 0.010, 0.015, 0.020) were prepared by conventional solid state reaction sintering technique. All ceramics present a pure perovskite phase structure, indicating that (Mn, Sb) has completely diffused into the BNBT6.5 lattice in the studied components. The addition of (Mn, Sb) disrupted the ferroelectric long–range order and promoted the electric field induced strain response. At x=0.015, a large electric field–induced unipolar strain of 0.48% (at an applied electric field of 80 kV/cm) with normalized strain d₃₃^*(S_max/E_max) of 602 pm/V are achieved. Temperature dependent measurements of both polarization and strain from room temperature to 120 °C were also studied, and the results suggest that the origin of the large strain is due to a reversible field–induced non–polar relaxor phase to polar ferroelectric phase transformation.     

Dielectric,ferroelectric and field-induced strain behavior of K0.5Na0.5NbO3-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

Lead-free piezoelectric (1 ? x)Bi_0.5(Na_0.78K_0.22)_0.5TiO₃–xK_0.5Na_0.5NbO₃ (BNKT–xKNN, x = 0–0.10) ceramics were synthesized using a conventional, solid-state reaction method. The effect of KNN addition on BNKT ceramics was investigated through X-ray diffraction (XRD), dielectric, ferroelectric and electric field-induced strain characterizations. XRD revealed a pure perovskite phase with tetragonal symmetry in the studied composition range. As the KNN content increased, the depolarization temperature (T_d) as well as maximum dielectric constant (?_m) decreased. The addition of KNN destabilized the ferroelectric order of BNKT ceramics exhibiting a pinched-type hysteresis loop with low remnant polarization (11 μC/cm²) and small piezoelectric constant (27 pC/N) at 3 mol% KNN. As a result, at x = 0.03 a significant enhancement of 0.22% was observed in the electric field-induced strain, which corresponds to a normalized strain (S_max/E_max) of ～434 pm/V. This enhancement is attributed to the coexistence of ferroelectric and non-polar phases at room temperature.     

Low temperature sintering of lead-free Bi0.5(Na0.82K0.18)0.5TiO3 piezoelectric ceramics by co-doping with CuO and Nb2O5

Effect of excess CuO additive on the sintering behavior and piezoelectric properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ ceramics was investigated. The addition of small amount of excess CuO as low as 1 mol% was quite effective to lower the sintering temperature (T_s) of BNKT ceramics down to 975 °C while their piezoelectric properties were degraded by Cu doping. However, the electric field-induced strain was markedly enhanced by further addition of Nb₂O₅ with CuO without elevating T_s. The normalized strain S_max/E_max of 427 pm/V was obtained with a specimen sintered with 0.02 mol CuO and 0.03 mol Nb₂O₅ in excess.     

A novel ((Bi0.5Na0.5)0.94Ba0.06)1-x (K0.5Nd0.5)xTiO3 lead-free relaxor ferroelectric ceramic with large electrostrains at wide temperature ranges

Novel ((Bi_0.5Na_0.5)_0.94Ba_0.06)_1-x(K_0.5Nd_0.5)_xTiO₃(x = 0.0, 0.02, 0.04, 0.06) lead-free ceramics (BNBT–xKN) were prepared by the solid-state reaction method. The effects of A-site (K_0.5Nd_0.5)²⁺ complex-ion substitution on their phase structure, dielectric, piezoelectric, and electromechanical properties were studied. The X-ray diffraction results indicate that all compositions are located in the morphotropic phase boundary (MPB) region where the tetragonal phase coexists with the rhombohedral phase. In addition, as the KN content increases, the ferroelectric order transform to relaxor order, which is characterized by a degeneration of maximum polarization, remnant polarization and correspondingly adjusts the ferroelectric-relaxor transformation temperature (T_F-R) to room temperature. Interestingly, the disruption of ferroelectric phase caused a significant improvement of strains. A maximum strain of ~ 0.52% corresponding to normalized strain of ~ 612 pm/V appeared at 85 kv/cm for the x = 0.04 composition. Particularly, the composition of x = 0.04 exhibited high electrostrains of temperature insensitivity, which remained above 0.4% and kept within 10% from ambient temperature up to 110 °C. It can be ascribed to the coexistence of non-ergodic and ergodic states in the relaxor region. As a result, the systematic investigations on the BNBT–xKN ceramics can benefit the developments of temperature-insensitive “on-off” actuators.     

Electric-field-temperature phase diagram and electromechanical properties in lead-free (Na0.5Bi0.5)TiO3-based incipient piezoelectric ceramics

In this work, the (1-x)(0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃)-xSrTiO₃ (NKBT-xST) incipient piezoelectric ceramics with x = 0–0.07 (0ST-7ST) were prepared by the solid-state reaction method and their structural transformation and electromechanical properties were investigated as a function of ST content. As the ST content increases, the long-range ferroelectric order is disrupted, and the ferroelectric-relaxor phase transition temperature (T_FR) shifts to around room temperature for NKBT-5ST ceramics, accompanied by a relatively high electrostrain of 0.3% at 6 kV/mm. The large strain response associated with the vanished ferroelectric properties around T_FR can be attributed to the reversible relaxor-ferroelectric phase transition. The electric-field-temperature (E-T) phase diagrams were established, and the transition between the two field-induced long-range ferroelectric states were found to take place via a two-step switching process through an intermediate relaxor state. The threshold electric field to trigger the conversion between ferroelectric state and relaxor state depends strongly on the dynamics of polarization relaxation, which is influenced by temperature and composition.     

The electrical properties of (1−x)(Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3)–xCaZrO3 lead-free piezoelectric ceramics

Lead-free (1−x)(0.0852Bi_0.5Na_0.5TiO₃–0.12Bi_0.5K_0.5TiO₃–0.028BaTiO₃)–xCaZrO₃ piezoelectric ceramics (BNT−BKT−BT−xCZ, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) were prepared by using a conventional solid-state reaction method. The effects of CZ-doping on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT−BKT−BT−xCZ system were systematically investigated. The polarization and strain behaviors indicated that the long-range ferroelectric order in the unmodified BNT−BKT−BT ceramics was disrupted by the increase of CZ-doping content, and correspondingly the depolarization temperature (T_d) shifted down from 109 °C to below room temperature. When x>0.03, accompanied with the drastic decrease in the remnant polarization (P_r) and piezoelectric coefficient (d₃₃), the electric-field-induced strain was enhanced significantly. A large unipolar strain of 0.35% under an applied electric field of 70 kV/cm (S_max/E_max=500 pm/V) was obtained in the BNT−BKT−BT−0.04CZ ceramics at room temperature, which was attributed to the reversible electric-field-induced phase transition between the relaxor and ferroelectric phases.     

Enhanced microstructure and electrical properties of Mn-modified Bi0.5(Na0.65K0.35)0.5TiO3 ferroelectric ceramics

Bi_0.5(Na_0.65K_0.35)_0.5TiO₃ (BNKT) and Mn-modified Bi_0.5(Na_0.65K_0.35)_0.5(Mn_xTi_1−x)O₃ (BNKMT-10³x), (x=0.0–0.5%) ferroelectric ceramics were synthesized by solid-state reaction method. Optimization of calcination temperature in Mn-doped ceramics was carried out for the removal of secondary phases observed in XRD analysis. BNKMT ceramics sintered at 1090 °C showed enhanced dielectric, piezoelectric and ferroelectric properties in comparison to pure BNKT. The average grain size was found to increase from 0.35 μm in BNKT to 0.52 μm in Bi_0.5(Na_0.65K_0.35)_0.5(Mn_0.0025Ti_0.9975)O₃ (BNKMT-2.5) ceramics. The dielectric permittivity maximum temperature (T_m) was increased to a maximum of 345 °C with Mn-modification. AC conductivity analysis was performed as a function of temperature and frequency to investigate the conduction behavior and determine activation energies. Significant high value of piezoelectric charge coefficient (d₃₃=176 pC/N) was achieved in BNKMT 2.5 ceramics. Improved temperature stability of ferroelectric behavior was observed in the temperature dependent P–E hysteresis loops as a result of Mn-incorporation. The fatigue free nature along with enhanced dielectric and ferroelectric properties make BNKMT-2.5 ceramic a promising candidate for replacing lead based ceramics in device applications.     

Effect of compositional variations on phase transition and electric field-induced strain of (Pb,Ba) (Nb,Zr, Sn,Ti)O3 ceramics

(Pb_0.97Ba_0.02)Nb_0.02(Zr_0.55Sn_0.45?xTi_x)_0.98O₃ (PBNZST, 0.03≤x≤0.06) ceramics were prepared by conventional solid state synthesis and their crystal structure, ferroelectric, dielectric, and electric field-induced strain properties were systemically investigated. A transformation from antiferroelectric (AFE) phase to ferroelectric (FE) phase was observed at 0.05<x<0.06. Besides, with the increase of Ti content, the electric field-induced strain decreased, due to the larger strain of AFE ceramics compared to FE ceramics. Further, when the measuring frequency decreased, the strain improved, because the electric field at low frequency allows a more efficient switching of domains, resulting in larger strain. The maximum strain of 0.55% was obtained in (Pb_0.97Ba_0.02)Nb_0.02(Zr_0.55Sn_0.45?xTi_x)_0.98O₃ antiferroelectric ceramics with x=0.03 at 2 Hz.     

Giant strain response and structure evolution in (Bi0.5Na0.5)0.945−x(Bi0.2Sr0.7□0.1)xBa0.055TiO3 ceramics

The microstructure, electric-field-induced strain, polarization, and dielectric permittivity in (Bi_0.5Na_0.5)_0.945−x(Bi_0.2Sr_0.7□_0.1)_xBa_0.055TiO₃ (BNBT–xBST) (0 ≤ x ≤ 0.08) electroceramics are investigated. An irreversible transition from rhombohedral and monoclinic coexistence phase to single rhombohedral phase is indicated with the remnant strain S_r = 0.330% at x = 0. As the BST content increases, the ferroelectric order is disrupted resulting in a degradation of the remnant polarization, coercive field, and the ferroelectric-to-relaxor transition temperature (T_F–R). The coexistence of ferroelectric relaxor and ferroelectric phase is observed for the optimum composition x = 0.02 at ambient temperature with a large strain of 0.428% at 60 kV/cm (normalized strain S_max/E_max = 713 pm/V). The large strain is contributed by both ferroelectric domain reorientation behavior and the reversible relaxor to ferroelectric phase transition.     

Large strain and relaxation behavior in CeO2 doped Bi0.487Na0.427K0.06Ba0.026TiO3 piezoceramics

xCeO₂-doped Bi_0.487Na_0.427K_0.06Ba_0.026TiO₃ lead-free piezoelectric ceramics (BNTC1000x, x=0, 0.3, 0.6, 0.8, 1.0, 1.2, 1.4 wt%), were synthesized by the solid-state reaction method. XRD patterns showed that all BNTC1000x ceramics exhibit pure single perovskite phase. At the critical composition BNTC12, a large electric-field-induced strain of 0.39% with normalized strain (S_max/E_max) of 561 pm/V was obtained under an electric field of 65 kV/cm. The ferroelectric phase was fully poled with electric field, and depoled once the applied electric field was removed. During that cycle, the non-180°-domains repeated switching and back-switching and the large strain was induced. The relaxation behavior was involved in BNTC1000x ceramics and induced by oxygen vacancy migration. Besides, this behavior was more predominant in BNTC12 than in BNTC0.     

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.     

Large electric field-induced strain in ternary Bi0.5Na0.5TiO3-BaTiO3-Sr2MnSbO6 lead-free ceramics

The large electric-field-induced strain of Bi_0.5Na_0.5TiO₃-BaTiO₃ based ceramic make it a potential replacement for lead-based ferroelectrics in actuator applications. Herein, a ternary system (1-x)(Bi_0.5Na_0.5)_0.935Ba_0.065TiO₃-xSr₂MnSbO₆ (BNBT6.5-xSMS) ceramic was fabricated using conventional solid-state reaction. It was found that the ferroelectric to relaxor phase transition temperature T_F-R gradually shifted to lower temperature by increasing SMS contents. The ferroelectricity and piezoelectricity of BNBT6.5 were highly affected by trace amount of SMS doping. For composition BNBT6.5-0.003SMS, where T_F-R was near room temperature, a large electric-field-induced unipolar strain of ~0.4% with high normalized strain (S_max/E_max) of 728 pm/V, which is comparable to lead-based ferroelectric/antiferroelectric ceramics, was achieved owing to the reversible electric-field-induced phase transition between a non-polar relaxor phase to a polar phase with long-range ferroelectric order.     

Phase structure,ferroelectric properties,and electric field-induced large strain in lead-free 0.99[(1−x)(Bi0.5Na0.5)TiO3-x(Bi0.5K0.5)TiO3]–0.01Ta piezoelectric ceramics

Lead-free 0.99[(1−x)(Bi_0.5Na_0.5)TiO₃-x(Bi_0.5K_0.5)TiO₃]–0.01Ta piezoelectric ceramics were prepared by a conventional solid-state reaction process. The ferroelectric properties, and strain behaviors were characterized. Increase of the (Bi_0.5K_0.5)TiO₃ content induces a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic phase. Accordingly, the ferroelectric order is disrupted significantly with the increase of (Bi_0.5K_0.5)TiO₃ content and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of 0.35% (corresponding to a large signal d₃₃ of 438 pm/V) in samples with 20 mol% (Bi_0.5K_0.5)TiO₃ content. Temperature dependent measurements of both polarization and strain from room temperature to 120 °C suggested that the origin of the large strain is due to a reversible field-induced nonpolar pseudocubic-to-polar ferroelectric phase transformation.     

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.     

Chemical composition and tolerance factor at the morphotropic phase boundary in (Bi0.5Na0.5)TiO3-based piezoelectric ceramics

A quantitative relation between the morphotropic phase boundary (MPB) composition and the tolerance factor (t) in (Bi_0.5Na_0.5)TiO₃ (BNT)-based piezoelectric ceramics was established. The t value of the MPB compositions in BNT-based ceramics is around 0.990–0.993 and is independent of the types of added compounds. In order to experimentally demonstrate it, two piezoelectric ceramic systems (1 ? x)(Bi_0.5Na_0.5)TiO₃–x(Ba_1?aSr_a)TiO₃, a = 0.05 and 0.3 (BNBST5-x and BNBST30-x, x < 12%), were used. X-ray diffraction patterns and the lattice parameter investigations revealed that these two systems formed solid solutions within the studied stoichiometry and showed a rhombohedral–tetragonal phase transformation. Furthermore, both the structure analysis and electric properties measurements indicated that the MPB compositions were BNBST5-6 and BNBST30-8 and their corresponding t value were 0.9900 and 0.9903, respectively. The results confirm the relation between the MPB composition and t value and provide a method for designing new piezoelectric materials.     

Structural properties of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric ceramics

A systematic XRD investigation of poled and unpoled ceramics of the system (1 ? x) Bi_0.5Na_0.5TiO₃–x BaTiO₃ (0 ≤ x ≤ 0.2) (BNBT) was performed. The variation of the lattice parameters confirms the existence of a morphotropic phase boundary at 0.06 ≤ x ≤ 0.08; however, significant differences in unit cell parameters between poled and unpoled states appear. Lattice distortions of the rhombohedral and tetragonal phases are significantly increased in poled samples. Dramatic changes in peak intensities of the pseudo-cubic (2 0 0) reflections between poled and unpoled samples reveal a strong enhancement of the tetragonal volume fraction in the poled state. Temperature-dependent XRD studies confirm a transition into a cubic high-temperature phase. This transition is rather smooth in the unpoled state. In poled samples, the tetragonal distortion and the tetragonal volume fraction display a different temperature variation and tetragonal regions seem to persist into the cubic phase field.     

Phase structure,piezoelectric, ferroelectric,and electric-field-induced strain properties of Nb-modified 0.8Bi0.5Na0.5TiO3−0.2Sr0.85Bi0.1TiO3 ceramics

0.8Bi_0.5Na_0.5Ti_(1-x)Nb_xO₃−0.2Sr_0.85Bi_0.1TiO₃ (BNT-SBT-xNb, x = 0.00, 0.01, 0.02, and 0.03) piezoelectric ceramics were prepared by traditional solid state reaction and the influence of Nb substitution on the phase structure, ferroelectric, piezoelectric, and electric-field-induced strain properties in BNT-SBT ceramics were studied. XRD results exhibited that Nb⁵⁺ ions could fully diffuse into BNT-SBT structure to form a solid solution when x = 0.01. P-E loops and S-E curves suggested that the ferroelectric phase transformed to ergodic relaxor state (FE-to-ER) with the increasing the amount of Nb additive, indicating the ferroelectric long-ranged order was disturbed by the excess of Nb. With increasing Nb doping, phase transition temperature from normal ferroelectric to ergodic relaxor (short for T_F-R) could be reduced from 120 °C to 40 °C. Furthermore, for sample with x = 0.01, the normalized strain d₃₃^* got a maximum value ~571 pm/V due to the phase transition from ergodic relaxor to ferroelectric (ER-to-FE) under electric field.     

Lead-free Bi1/2(Na0.82K0.18)1/2TiO3 relaxor ferroelectrics with temperature insensitive electrostrictive coefficient

The electric field-induced strain of Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ (BNKT) ceramics modified with BaZrO₃ (BZ) was investigated as a function of composition and temperature. Unmodified BNKT ceramics revealed a typical ferroelectric butterfly-shaped bipolar S–E loop at room temperature, whose normalized strain (S_max/E_max) showed a significant temperature coefficient of 0.38 pm/V/K. As the BZ content increased in the solid solution up to 5 mol%, the ferroelectric BNKT gradually transformed to a relaxor. Finally, 5 mol% BZ-modified BNKT ceramics showed a typical electrostrictive behavior with a thermally stable electrostrictive coefficient (Q₃₃) of 0.025 m⁴/C², which is comparable to that of Pb(Mg_1/3Nb_2/3)O₃ (PMN) ceramics that have been primarily used as Pb-based electrostrictive materials.     

Creating adjoining polymorphic phase transition (PPT) regions in ferroelectric (Ba,Sr)(Zr,Ti)O3 ceramics

In this work, we report the polymorphic phase transitions(PPT) in ferroelectric Ba_0.95Sr_0.05Zr_xTi_(1-x)O₃ (BSZT, x = 0.01–0.10) ceramics synthesized by using a solid-state reaction method. The doping elements and composition ratios were selected to create adjoining PPT phase boundaries near room temperature, hence to achieve a broadened peak of piezoelectric performance with respect to composition. The temperature-composition phase diagram was constructed and the effects of PPT on the electromechanical and ferroelectric properties of the ceramics were investigated. It was revealed that the two adjacent PPT regions at room temperature showed different characteristics in property enhancement. However, due to the proximity of the phase boundaries, Ba_0.95Sr_0.05Zr_xTi_(1-x)O₃ ceramics in a fairly broad range of compositions (0.02 ≤ x ≤ 0.07) showed excellent piezoelectric properties, including a large piezoelectric constant (312 pC/N ≤ d₃₃ ≤ 365 pC/N) and a high electromechanical coupling coefficient k_p (0.42 ≤ k_p ≤ 0.49).     

Processing and enhanced electrical properties of Sr1-x(K0.5Bi0.5)xBi2Nb2O9 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics, Sr_1−x(K_0.5Bi_0.5)_xBi₂Nb₂O₉ (SKBN-x, x=0, 0.2, 0.5, 1.0), were synthesized by a conventional solid-state reaction. Structural and electrical properties of SKBN-x ceramics were investigated. X-ray diffraction analysis suggested that the substitution led to the formation of a layered perovskite structure. Plate-like morphologies for the grains were clearly observed in all the samples, which are characteristic for layer-structure Aurivillius compounds. The Curie temperature (T_c) is found to shift to higher temperature from 445 °C to 509 °C with increasing (K, Bi) content. Excellent remanent polarization (2P_r∼15 μC/cm²) were obtained for SKBN-0.2 ceramic. High piezoelectric coefficient of d₃₃∼21  pC/N were obtained for the samples at x=0.5. Additionally, thermal annealing studies indicated that the piezoelectric coefficient (d₃₃) of SKBN-0.5 was unchanged even if annealing temperature increased to be 450 °C, demonstrating the ceramics are the promising candidates for high-temperature applications.