Structural,dielectric and piezoelectric properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Zn0.5Ti0.5)O3 thin films prepared by sol–gel method

Lead free (1−x)(0.8Bi_0.5Na_0.5Ti_0.5O₃–0.2Bi_0.5K_0.5TiO₃)–xBiZn_0.5Ti_0.5O₃ (x=0–0.06) (BNT–BKT–BZT) thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel processing technique. The effects of BZT content on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT–BKT–BZT thin films were investigated systematically. The BNT–BKT–BZT thin films undergo a transition from ferroelectric to relaxor phase with increasing temperature. The phase transition temperature decreases with the increase of BZT content. The BNT–BKT–BZT thin film with x=0.04 exhibits the best ferroelectric properties (P_max=40 µC/cm² and P_r=10 µC/cm²), largest dielectric constant (ε=560) and piezoelectric constant (d₃₃=40 pm/V). This finding demonstrates that the BNT–BKT–BZT thin film has an excellent potential for demanding high piezoelectric properties in lead free films.     

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.     

Morphotropic phase boundary and electrical properties of lead-free (K0.5Bi0.5)TiO3–Bi(Ni0.5Ti0.5)O3 relaxor ferroelectric ceramics

Lead-free relaxor ferroelectric ceramics (1?x)(K_0.5Bi_0.5)TiO₃–xBi(Ni_0.5Ti_0.5)O₃ were prepared by a conventional solid-state route, the phase transition behavior and corresponding electrical properties were investigated. A typical morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified to be in the range of 0.05<x<0.07 where the optimum piezoelectric and electromechanical properties of d₃₃=126 pC/N and k_P=18% were achieved. Most importantly, a high Curie temperature ~320 °C, around which the material shows a typical relaxor ferroelectric behavior characterized by the presence of diffuse phase transition and frequency dispersion, was obtained in MPB compositions, significantly higher than those of some existing MPB lead-free titanate systems. These results demonstrate a tremendous potential of the studied system for device applications.     

Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–BaTiO3–SrTiO3陶瓷的准同型相界与电性能

采用固相法制备了 Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–BaTiO3–SrTiO3（NBT–KBT–BT–ST）陶瓷,该体系是按（1–2x）（0.8NBT–0.2KBT）–x（0.94NBT–0.06BT）–x（0.74NBT–0.26ST） （x = 0.10、0.20、0.25、0.30、0.35、0.40、0.45）组合而成的,研究了该系陶瓷的结构与电性能。结果表明：所有样品都处于三方–四方准同型相界区域。该系陶瓷在准同型相界附近表现出了优异的压电性能,压电常数 d33、机电耦合系数 kp和剩余极化强度 Pr随 x 的增加先升高后降低,其中 x=0.35 陶瓷的电性能最佳：d33= 210 pC/N,kp= 0.319,Pr= 39.3 μC/cm2,Ec= 20.2 kV/cm,是一种良好的无铅压电陶瓷候选材料。依据准同型相界组成的线性组合规律来寻找具有优异压电性能的 NBT–KBT–BT–ST 陶瓷准同型相界组成是可行的。     

Enhanced energy storage properties of BiAlO3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free antiferroelectric ceramics

Lead-free (1−x)(0.75Bi_0.5Na_0.5TiO₃–0.25Bi_0.5K_0.5TiO₃)–xBiAlO₃ (BNT–BKT–100xBA, x=0–0.10) ceramics were prepared by two-step sintering method and their phase structure, micro morphology and electrical properties were systematically investigated. X-ray diffraction analysis indicates a pure perovskite phase for x≤0.06 as well as a structural evolution from a tetragonal toward a pseudocubic phase. Transmission electron microscopy study of the x=0.04 composition reveals the existence of antiferroelectric phase with a⁰a⁰c⁺ oxygen octahedron tilting which is in the form of nano-domains. Polarization-electric field and current-electric field hysteresis loops demonstrate that the increase of BA concentration destroys the ferroelectric order and strengthens antiferroelectric order. A much enhanced energy storage density of 1.15 J/cm³ and efficiency of 73.2% is achieved under 105 kV/cm at x=0.06. In addition, its energy storage property is found to depend weakly on temperature within the measurement range of 25–150 °C.     

Phase–composition and temperature dependence of electrocaloric effect in lead–free Bi0.5Na0.5TiO3–BaTiO3–(Sr0.7Bi0.2□0.1)TiO3 ceramics

The (0.94–x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT, 0 ≤ x ≤ 0.24) solid solution ceramics were synthesized via a conventional solid–state reaction method and the correlation of phase structure, piezoelectric, ferroelectric properties and electrocaloric effect (ECE) was investigated in detail. The ECE in lead–free BNT–BT–xSBT ceramics was measured directly using a home–made adiabatic calorimeter with maximum adiabatic temperature change ΔT = 0.4 K with x = 0.08 under the electric field E = 6 kV/mm at room temperature. The position of maximum ECE was found in the vicinity of nonergodic and ergodic phase boundary, where the maximum change in entropy occurs as a result of the field–induced phase transformation between the ergodic and long–range ferroelectric phase. Besides, the mechanism for the shift of ECE peak is discussed in detail. Finally, the temperature dependence of ECE for BNT–BT–xSBT (x = 0, 0.04 and 0.08) was also investigated. This work may present a guideline for designing BNT–based ferroelectric relaxor ceramics for EC cooling technologies.     

La掺杂(Na0.5Bi0.5)0.82(K0.5Bi0.5)0.18TiO3系统无铅压电陶瓷的制备工艺探讨

采用传统陶瓷工艺，研究了制备[（Na0.5Bi0.5）0.82（K0.5Bi0.5）0.18]1-xLaxTiO3（x=0．00，0．01，0．03，0．05，0．10）无铅压电陶瓷的工艺条件对陶瓷的物相组成、显微结构和压电性能的影响。利用XRD、SEM等技术分析结果表明，合成温度的提高有利于主晶相的形成，且此系统烧成温度范围较窄，故需控制在合适的烧成温度下才能得到高致密度的陶瓷。同时，研究了极化工艺条件对材料压电性能的影响，结果表明，提高极化电场强度、控制适当的极化温度有利于提高材料的压电性能。     

Field-induced large strain in lead-free 0.99[(1−x) Bi0.5(Na0.80K0.20)0.5TiO3–xBiFeO3]–0.01(K0.5Na0.5)NbO3 piezoelectric ceramics

Lead-free 0.99[(1−x) Bi_0.5(Na_0.80K_0.20)_0.5TiO₃–xBiFeO₃]–0.01(K_0.5Na_0.5)NbO₃ (BNKT20–100xBF–1KNN) piezoelectric ceramics were fabricated through conventional techniques. Results showed that changes in BF content of BNKT20–100xBF–1KNN induced transition from the ferroelectric phase to the ergodic relaxor phase. These changes also significantly disrupted long-range ferroelectric order, thereby correspondingly adjusting the ferroelectric-relaxor transition point T_F-R to room temperature. A large strain of 0.39% at the electric-field of 80 kV/cm (corresponding to a large signal d₃₃^* of 488 pm/V) was obtained at x=0.06, which originated from the composition proximity to the ferroelectric-relaxor phase boundary. Moreover, the high-strain material exhibited exceptional fatigue resistance (up to 10⁶ cycles) as a result of the reversible field-induced phase transition. The proposed material exhibits potential for novel ultra-large stroke and nonlinear actuators that require enhanced cycling reliability.     

溶胶-凝胶法合成(Na0.5Bi0.5)TiO3微粉

以钛酸四丁酯、硝酸铋、醋酸钠和冰醋酸为原料,利用溶胶-凝胶工艺得到透明凝胶,经干燥后煅烧成(Na0.5Bi0.5)TiO3微粉。通过对溶胶体系水/醇盐的摩尔比、初始pH值及胶凝温度对(Na0.5Bi0.5)TiO3凝胶体系溶胶-凝胶形成过程影响的研究,发现水/醇盐比R在35≤R≤60,pH在2.2~3.5,反应温度在40~60℃时,能够得到透明的溶胶;通过TG-DTA、SEM、X-ray等分析手段对(Na0.5Bi0.5)TiO3粉体进行测试,表明在650℃合成1h可以得到单一钙钛矿(Na0.5Bi0.5)TiO3晶体;采用TEM对(Na0.5Bi0.5)TiO3干凝胶粉体分析其粒径大小约为10nm。     

Temperature-independent permittivity of xBaTiO3–(1−x)(0.5Bi(Mg1/2Ti1/2)O3–0.5BiScO3) ceramics

xBaTiO₃–(1−x)(0.5Bi(Mg_1/2Ti_1/2)O₃-0.5BiScO₃) or xBT–(1−x)(0.5BMT–0.5BS) (x=0.45–0.60) ceramics were prepared by using the conventional mixed oxide method. Perovskite structure with pseudo-cubic symmetry was observed in all the compositions. Dielectric measurement results indicated that all the samples showed dielectric relaxation behavior. As the content BaTiO₃ was decreased from 0.60 to 0.45, temperature coefficient of permittivity (TCε) in the range of 200–400 °C was improved from −706 to −152 ppm/°C, while the permittivity at 400 °C was increased from 1208 to 1613. The temperature stability of permittivity was further improved by using 2 mol% Ba-deficiency. It was found that lattice parameter and grain size of the 2 mol% Ba-deficient ceramics were smaller than those of their corresponding stoichiometric (S) counterparts, with TCε in the range of 200–400 °C to be improved noticeably. For example, TCε of the Ba-deficiency sample with x=0.45 was −75 ppm/°C in the temperature range of 200–400 °C and the permittivity was 1567 at 400 °C. The results obtained in this work indicated that xBT–(1−x)(0.5BMT–0.5BS) ceramics are very promising candidates for high temperature capacitor applications.     

Phase transition behavior and electrical properties of lead-free (Bi0.5K0.5)TiO3–LiNbO3 relaxor ferroelectric ceramics

(1?x)(Bi_0.5K_0.5)TiO₃–xLiNbO₃ ((1?x)BKT–xLN) lead-free relaxor ferroelectric ceramics were prepared by a conventional solid-state route and their phase transition behavior and the corresponding electrical properties were investigated. A morphotropic phase boundary separating rhombohedral and tetragonal phases was identified in the composition range of 0.015<x<0.03, where the improved electrical properties of piezoelectric constant d₃₃=75 pC/N and electromechanical coupling factor k_p=0.18 were obtained. Moreover, all samples show typical relaxor behavior characterized by the presence of diffuse phase transition and frequency dispersion. It was found that the dielectric relaxation behavior of BKT ceramics can be obviously enhanced with the addition of LN. In addition, the effect of the LN addition on the ferroelectric properties was also investigated by measuring polarization versus electric field hysteresis loops.     

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.     

(Na1-xKx)0.5Bi0.5TiO3系的离子挥发性研究

利用固相法制备了(Na1-xKx)0.5Bi0.5TiO3系压电陶瓷,研究其中Bi3 、Na 、K 离子的挥发对其性能的影响。研究结果表明Bi3 的挥发性对样品的性能影响较大,而Na 、K 离子相对较小。     

Analysis of the rhombohedral–tetragonal symmetries coexistence in lead-free 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics from nanopowders

Ceramics with perovskite-type structure and 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (BNBT) composition have been studied by conventional powder X-ray diffraction in Bragg–Brentano geometry. Ceramics were obtained from sol–gel autocombustion nanopowders and processed either by hot pressing and subsequent recrystallisation or pressureless sintering in two steps. These methods provided single-phase, sub-micron grain size (<700?nm), dense ceramics with good piezoelectric performance (96–94% of theoretical density and d₃₃?=?143–124?pC?N^–1, respectively). For the considered ceramics, the splitting of the peaks of the cubic perovskite-type structure with 111 and 200 Miller indices has been repeatedly used as a symmetry identification criterion. In this work a simple, yet powerful, procedure to validate the consistency of the mentioned splitting interpretation is presented. Based on peaks fitting and well-known crystallographic expressions, the rhombohedral and tetragonal symmetries' coexistence is verified. The suggested procedure can be applied to the study of peak splitting in ceramics at Morphotropic Phase Boundaries in a general way. In a given series of BNBT ceramics, inconsistencies for interplanar distances, intensities' ratios and the evolution of these from not-poled to poled samples have been found. In poled ceramics, special care has been taken when carrying out this analysis, due to the anisotropic strains arising from ferroelectric (FE) domain orientation. Poling gives rise to a displacement of the peaks angular positions and modification of the intensity ratios. However, the interplanar distance changes associated with the angular deviations here observed are one order of magnitude higher than those expected from anisotropic strains. These results set up a doubt on the sufficiency of the [rhombohedral?+?tetragonal] model to characterise the considered ceramics. A model of a mesoscopic FE phase with rhombohedral symmetry, a mesoscopic and globally weakly polar phase, with cubic symmetry, and a nanosised phase, also cubic, is presented as a plausible alternative.     

Ferroelectric and magnetic properties of magnetoelectric (Na0.5Bi0.5)TiO3–BiFeO3 synthesized by acetic acid assisted sol–gel method

The perovskite solid solution system resulting from the combination of Na_0.5Bi_0.5TiO₃ (NBT) and BiFeO₃ (BFO) has been studied for its magnetoelectric properties in BFO-rich phases of the series, using magnetometry and piezoresponse force microscopy (PFM). Raman spectroscopy results confirmed the successful formation of solid solutions having spectral features that are combination of two parent compounds. The solid solutions displayed a variable bandgap in the range 2.12–2.39 eV with composition. The results of PFM suggest an improved spontaneous polarization with the progressive replacement of (Na,Bi)²⁺ and Ti⁴⁺ ions at A- and B-sites of BFO lattice. At the same time room temperature weak ferromagnetic response was seen in mixed compositions. A systematic shift in magnetic transition temperatures from 650 K to 410 K was observed with the increase of NBT content. Magnetoelectric (ME) coefficient of 9 mV/cm-Oe was obtained for 40NBT–60BFO composition. Our studies indicate that NBT–BFO solid solution system is a potentially useful candidate for lead-free single phase ME applications.     

Effect of WO3 doping on dielectric and ferroelectric properties of 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics

WO₃(0–6 mol%)-doped 0.94Bi_0.5Na_0.5TiO₃–0.06BaTiO₃ lead-free ceramics were synthesized by conventional solid-state reaction. The effect of WO₃ addition on the structure and electrical properties were investigated. The result revealed that a small amount of WO₃ (≤1 mol%) can diffuse into the lattice and does not significantly affect the phase structure, however, more addition will result in distortion and enlargement of the unit cells. The maximum permittivity temperature (T_m) is suppressed dramatically as the dopant increasing, while the depolarization temperature (T_d) fall to the minimum with 1 mol% WO₃ additive. The remanent polarization (P_r) was enhanced and coercive field (E_c) was reduced by doping with WO₃. The strain shows the largest value for 1 mol% doped sample, which is due to a field-induced antiferroelectric–ferroelectric phase transition.     

Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3系无铅压电陶瓷的制备工艺研究

利用XRD、SEM等分析技术 ,研究了Na0 .5Bi0 .5TiO3 -K0 .5Bi0 .5TiO3 系无铅压电陶瓷的合成温度 ,烧成工艺条件对陶瓷晶体结构、压电性能的影响。结果表明 ,合成温度提高有利于主晶相的形成 ,适当延长保温时间有利于材料的压电性能。该体系随着KBT含量的增加 ,烧结温度提高 ,烧结温度范围变窄。同时研究了极化工艺条件对材料压电性能的影响表明 ,提高极化电场和适当提高极化温度有利于压电性能的提高 ,但过高的温度由于受到材料高温下退极化的影响而导致材料压电性能变差