Improvement of the piezoelectric and ferroelectric properties of (K,Na)0.5NbO3 ceramics via two-step calcination–milling route

A second calcination–milling step was introduced in the conventional processing of (K, Na)_0.5NbO₃ (KNN) ceramics (sintered in air) to further homogenenize the particle size distribution of the pre-sintered powders. The ceramic derived from the powders prepared by the two-step route possesses grains with better uniformity and is more compact. The relative density of the bulk ceramic reached 96.9%. Excellent properties are obtained in as-prepared KNN ceramics with k_p=44%, d₃₃=111 pC/N, tanδ=0.85%, ε₃₃^T/ε_o=311, Q_m=193, P_r=25.4 μC/cm², d₃₃^⁎=251 pm/V, which are superior to those of the ceramics derived from the powders calcined once as used in the traditional processing. These results indicate that twice-calcination–milling route is shown to be a facile and effective way to simultaneously improve the piezoelectric and ferroelectric properties of KNN ceramics without sintering aids.     

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.     

A novel Na0.5K0.5NbO3–La(Zn0.5Ti0.5)O3 ceramics with excellent dielectric properties at wide temperature range

Ceramic-based dielectrics are considered as the best candidates for high temperature capacitors because of their outstanding mechanical and electrical properties. Nevertheless, conventional barium titanate-based capacitors show narrow operating temperature ranges owing to the low tetragonal-cubic phase transition temperature. In order to increase the working temperature and relative permittivity, a novel (1-x)Na_0.5K_0.5NbO₃- xLa(Zn_0.5Ti_0.5)O₃ (NKN-xLZT) ceramics were chosen to meet the targets in this work. The NKN-xLZT ceramics with sub-micrometer grains (0.2–0.4 μm) were synthesized via a conventional solid-state sintering route. A relative permittivity (ε’ = 1560 ± 15%) with low loss tangent over wide temperature range from 96 °C to 350 °C was obtained in the x = 0.02 ceramics. Additionally, the crystal structure distortion and conduction behaviors of the NKN-xLZT ceramics were systematically studied. The decrease of oxygen octahedron distortion induced a weak polarization, and the high resistance (9 × 10⁶ Ωcm at 400 °C) greatly suppressed the long-term migration of defective ions in the ceramics. Therefore, the low loss tangent and high permittivity were still stabilized at the high temperature. It believes that the NKN-xLZT ceramic system in this work will become one of the most promising candidates for high-temperature capacitor devices.     

Structure,dielectric and ferroelectric properties of 0.92Na0.5Bi0.5TiO3–0.06BaTiO3–0.02K0.5Na0.5NbO3 lead-free ceramics: Effect of Co2O3 additive

0.92Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–0.02K_0.5Na_0.5NbO₃+x wt% Co₂O₃ (NBKT–xCo, x=0, 0.2, 0.4, 0.6, 0.8) lead-free ferroelectric ceramics were prepared via a conventional solid state reaction method. Effects of Co₂O₃ additive on crystallite structure, microstructure, dielectric and ferroelectric properties of the NBKT–xCo ceramics were studied. X-ray diffraction results showed that the rhombohedral–tetragonal morphotropic phase boundary existed in all the ceramics, with relative amount of tetragonal phase varying with the content of Co₂O₃. Average grain size, maximum value of dielectric constant, Curie temperature and ferroelectric properties of the ceramics were close related to the content of Co₂O₃. The dielectric anomaly caused by the phase transition between the ferroelectric phase and the so-called “intermediate phase” was observed in the ceramics with x≤0.2, while it disappeared with further increasing x. All the ceramics showed a diffuse phase transition between the “intermediate phase” and the paraelectric phase. The change in the ferroelectric properties with changing the content of Co₂O₃ was discussed by considering the competitive effects among grain size, relative amount of the tetragonal phase and oxygen vacancies.     

Enhanced temperature stability in the O–T phase boundary of (K,Na)NbO3-based ceramics

In order to obtain excellent electrical properties and its temperature stability of KNN-based ceramics to meet the practical applications, a new lead-free material system of (1−x)K_0.5Na_0.5Nb_0.96Sb_0.04O₃-xBi_0.5Na_0.5Zr_0.8Sn_0.2O₃ (KNNS-xBNZS, 0 ≤ x ≤ 0.060) was designed, and the enhanced electrical properties (eg, d₃₃ ~ 465 pC/N, ε_r ~ 3318, S_uni ~ 0.133%) is obtained in the ceramics with x = 0.04. The physical origin of enhanced electric properties should be ascribed to the phase instability of R-T, resulting in a low-energy barrier, which can greatly facilitate the polarization switching. Moreover, the temperature stable of piezoelectric constant (d₃₃ or d₃₃*) is measured by three distinctive methods in different phase boundaries (O, O-T, R-T) at the temperature range 20°C-180°C, revealing that the O-T phase boundary has a relatively good temperature stability. A mode is used to show how to effectively modified the piezoelectric constant (d₃₃ or d₃₃*) and its temperature stability, we believe that such a strategy may further improve the temperature stability of d₃₃ or d₃₃* value in KNN-based ceramics.     

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.     

Enhancing the dielectric and energy storage properties of lead-free Na0.5Bi0.5TiO3–BaTiO3 ceramics by adding K0.5Na0.5NbO3 ferroelectric

A novel strategy of enhancing the dielectric and energy storage properties of Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBT–BT) ceramics by introducing a K_0.5Na_0.5NbO₃ (KNN) ferroelectric phase is proposed herein, and its underlying mechanism is elucidated. The lead-free KNN ceramic decreases the residual polarisation and increases the electric breakdown strength of the NBT–BT matrix through the simultaneous modification of its A-sites and B-sites. The obtained NBT?BT?x?KNN ceramics have a perovskite structure with unifying grains. A bulk 0.9NBT–BT–0.1KNN ceramic sample with a thickness of 0.2 mm possesses a high energy storage density of 2.81 J/cm³ at an applied electric field of 180 kV/cm. Moreover, it exhibits good insulation properties and undergoes rapid charge and discharge processes. Therefore, the obtained 0.9NBT–BT–0.1KNN ceramic can be potentially used in high-power applications because of its high energy density, good insulation properties, and large discharge rate.     

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.     

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.     

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 temperature stability and fatigue-resistant behavior in MgTiO3-doped 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 lead-free ceramics

0.948(K_0.5Na_0.5)NbO₃–0.052LiSbO₃–xMgTiO₃ (x=0, 0.005 and 0.010) (abbreviated as KNLNS–xMT) lead-free piezoelectric ceramics were prepared by normal sintering. The effect of MT addition on KNLNS ceramics was investigated through dielectric, ferroelectric and electric field-induced strain characterizations. The grain size decreased slightly after the addition of MT, and more uniform grains were obtained. Impedance measurements made over a wide range of temperatures (425–525 °C) showed the presence of both bulk and grain boundary effects in the materials. The activation energies E_a were 0.483 and 0.507 eV for KNLNS and KNLNS–0.005MT ceramics respectively, indicating that the conduction process was due to oxygen vacancies in the higher temperature region. The P_r and unipolar strain of the MT modified ceramics exhibited lower temperature sensibility than KNLNS ceramics in the temperature range 30–120 °C. Meanwhile, the MT doped samples showed less degradation in both switchable polarization and unipolar strain after 10⁶ switching cycles than those of KNLNS. It is expected that the KNLNS–xMT ceramics is promising candidate for lead-free piezoceramics and could be used in practical applications.     

Room-temperature ferromagnetism in (K0.5Na0.5)NbO3-xBaNi0.5Nb0.5O3-δ ferroelectric ceramics with narrow bandgap

In this paper, a simple, reproducible and cost-effective solid-state reaction sintering process is developed to fabricate (K_0.5Na_0.5)NbO₃-xBaNi_0.5Nb_0.5O_3-δ (KNN-xBNN) ceramics with a narrow bandgap and room-temperature ferromagnetism. Here, we report a systematic investigation of the influence of the BaNi_0.5Nb_0.5O_3-δ (BNN) concentration on the properties of KNN-xBNN ceramics. All ceramics form orthorhombic perovskite structures with a space group Amm2 and a weak peak at the wavelength of 550 cm^?1 that is characteristic of the pillow shoulder of the orthorhombic phase. KNN-xBNN ceramics with x between 0.02 and 0.08 have a narrow bandgap of about 2.5 eV—much smaller than the 3.5 eV of its parent (K_0.5Na_0.5)NbO₃ (KNN) ceramic—which is attributed to Ni²⁺-oxygen vacancy combinations (Ni²⁺-V_O) raising the valence electron energy level of the KNN ceramic. Furthermore, doping BNN into KNN ceramics can significantly convert the magnetism from diamagnetism to ferromagnetism and the component of x = 0.08 achieves both maximum saturation magnetisation intensity (14 memu/g) and minimum coercive magnetic field (80 Oe). Our findings provide a systematic insight into the bandgap tunability and ferromagnetism induction at room temperature in lead-free perovskite KNN-xBNN ceramics, as well as demonstrate their potential applications in perovskite solar cells and multiferroic devices.     

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.     

Hardening behavior and highly enhanced mechanical quality factor in (K0.5Na0.5)NbO3–based ceramics

This paper relates the microstructure, crystallographic structure, ferroelectric, and piezoelectric properties of (K_0.5Na_0.5)NbO₃ (KNN) ceramics modified with 0.38 mol% K_5.4Cu_1.3Ta₁₀O₂₉ (KCT) and different amounts of CuO. Results revealed that the addition of KCT and CuO were effective in enhancing the sinterability of KNN. The internal bias field (E_ib) increased from 0.3 kV/mm to 0.58 kV/mm at 0.5 mol% CuO–added KNN+KCT ceramics. The increase of E_ib corresponds very well with the observed increase of the mechanical quality factor (Q_m) from 112 to 2665 for 0.5 mol% CuO. Besides, addition of 0.5 mol% CuO to KNN+KCT resulted in a large increase of the EPR signal, which is related to the increased amount Cu²⁺ and a corresponding increase of the concentration of defect dipoles. This result is in good agreement with the increased E_ib and the resulting hardening behavior.     

Effects of sintering temperature on properties of Na0·5K0·5NbO3-LiSbO3-BiFeO3 lead free ceramics

Abstract

Abstract

Lead free 0·95K_0·5Na_0·5NbO₃-0·05LiSbO₃ (KNN-LS) ceramics doped with 0·4?mol.-% BiFeO₃ (BF) have been prepared by the conventional mixed oxide method with sintering temperature at 1065-1135°C in this paper. The samples are characterised by X-ray diffraction analysis and scanning electron microscopy. The dielectric and piezoelectric properties are also investigated. The results present that initially the increase in the sintering temperature is very effective in improving the density and electric properties. However, the properties of the samples would be deteriorated as they are sintered over the optimum temperature. The KNN-LS (doped with 0·4%BF) ceramics shows excellent properties with sintering temperature at 1100°C.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.     

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.     

Effect of SrTiO3 content on the growth of (100−x)(K0.5Na0.5)NbO3–xSrTiO3 lead-free piezoelectric single crystals grown by the solid-state crystal growth method

In the present work, the single crystal growth by solid-state crystal growth of (100?x)(K_0.5Na_0.5)NbO₃–xSrTiO₃, where x?=?0,1,2,3?mol-%, has been examined in order to study the effect of SrTiO₃ content on single crystal growth. Powders were prepared by the conventional mixed oxide method. <001> KTaO₃ seed crystals were buried in the powders, pressed into pellets and sintered at 1100°C for 1, 3, 5 and 10?h. Single crystals of the ceramic compositions grew onto the seeds. For the (K_0.5Na_0.5)NbO₃ sample, both single crystal growth and abnormal grain growth in the matrix began to take place within 1?h. As the amount of SrTiO₃ increased, the onset of both single crystal growth and abnormal grain growth were delayed. The effect of SrTiO₃ addition on the single crystal and matrix grain growth behaviour is explained in terms of the mixed control theory of grain growth.