Microstructure and piezoelectric properties of textured (Na0.84K0.16)0.5Bi0.5TiO3 lead-free ceramics

Textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were fabricated by reactive-templated grain growth in combination with tape casting. The effects of sintering conditions on the grain orientation and the piezoelectric properties of the textured (Na,K)_0.5Bi_0.5TiO₃ ceramics were investigated. The results show that the textured ceramics have microstructure with plated-like grains aligning in the direction parallel to the casting plane. The ceramics exhibit {h 0 0} preferred orientation and the degree of orientation is larger than 0.7. The degree of grain orientation increases with the increasing sintering temperature. The textured ceramics show anisotropy dielectric and piezoelectric properties in the directions of parallel and perpendicular to the casting plane. The ceramics in the perpendicular direction exhibit better dielectric and piezoelectric properties than those of the nontextured ceramics with the same composition. The optimized sintering temperature is 1150 °C where the maximum d₃₃ of 134 pC/N parallel to casting plane, the maximum k₃₁ of 0.31, and the maximum Q_m of 154 in perpendicular direction were obtained.     

Highly textured (Na1/2Bi1/2)0.94Ba0.06TiO3 ceramics prepared by the screen-printing multilayer grain growth technique

The screen-printing multilayer grain growth (MLGG) technique was successfully applied to perovskite-structured lead-free piezoelectric ceramics. Highly textured (Na_1/2Bi_1/2)_0.94Ba_0.06TiO₃ ceramics with (1 0 0) orientation were firstly fabricated by MLGG method with (or without) template particles. The MLGG approach using anisotropic Bi₄Ti₃O₁₂ templates resulted in >90% grain orientation, whereas the same approach without template particles resulted in high orientation degree. The grain orientation mechanism of MLGG using screen-printing was different form that of tape-casting and extrusion in templated grain growth (TGG) and reactive templated grain growth (RTGG) techniques. The interface between adjacent layers, which were formed by screen-printing, was the main mechanism for the texture development in MLGG technique. Compared with other grain orientation techniques, screen-printing was a simple, inexpensive and effective method to fabricate grain oriented lead-free piezoelectric ceramics.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

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.     

Enhanced piezoelectric properties and temperature-insensitive strain behavior of <001>-textured KNN-based ceramics

In this study, <001>-textured 0.99(K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃−0.01CaZrO₃ [abbreviated as 0.99KNLNS-0.01CZ] lead-free ceramics were prepared by templated grain growth (TGG) using plate-like NaNbO₃ templates and sintered by a two-step sintering process with different soaking time. All textured samples with high Lotgering factor (f >85%) presented orthorhombic and tetragonal coexisting phase, and the proportion of orthorhombic phase was varied with prolonged soaking time. A large piezoelectric constant d₃₃ (~ 310 pC/N) was obtained in the textured samples with a 12 h soaking time, which was almost twice larger compared to the randomly oriented one. Furthermore, the field-induced piezoelectric strain coefficient d₃₃^*(~ 440 pm/V) of the textured ceramics with 6 h soaking time was larger than the value of randomly oriented one (~ 298 pm/V) at room-temperature. Enhanced piezoelectric response and good temperature stability prove that <001>-textured 0.99KNLNS-0.01CZ ceramics are promising candidates in the field of lead-free piezoelectric materials.     

Low-temperature sintering and electrical properties of (K,Na)NbO3 based lead-free ceramics with high Curie temperature

Lead-free ceramics (1 ? x)(K_0.48Na_0.52)NbO₃–(x/5.15)K_2.9Li_1.95Nb_5.15O_15.3 (x = 0.3–0.6, KNN–KLN100x) were prepared by conventional sintering technique at a low temperature of 960 °C. The effects of KLN contents on microstructure, dielectric, and piezoelectric properties were investigated. After the addition of KLN, the sintering performance and Curie temperature of the ceramics were markedly improved. The ceramics with x = 0.3 exhibited very good piezoelectric properties: d₃₃ = 138 pC/N, k_p = 45.03%, T_c = 495 °C, the dielectric constant at room temperature ?_r (RT) = 478 and the maximum dielectric constant ?_r (max) = 5067. These results indicated that the KNN–KLN100x lead-free ceramics sintered at low temperatures are promising for high temperature piezoelectric applications.     

Microstructure design of lead-free piezoelectric ceramics

Computational and experimental methodologies are integrated into a novel combined technique to define microstructure design criteria and maximize the properties of rhombohedral Bi_0.5Na_0.4K_0.1TiO₃, from untextured (1 MRD), d₃₃ = 155 pC/N, to textured (4.41 MRDs), d₃₃ = 227 pC/N. Two-dimensional orientation maps obtained using electron backscatter diffraction on sequential parallel layers are used to computationally reconstruct three-dimensional samples, simulate the local piezoelectric grain interactions, and thus demonstrate that superior lead-free piezoelectric microstructures can be fabricated by engineering its associated crystallographic and polarization texture. Computer-generated material representations, based on the experimentally determined microstructures, were used to simulate the crystallographic orientation of each grain, as function a macroscopic polarization and crystallographic texture. Computer-generated material representations, based on the experimentally determined microstructures, were used to simulate the crystallographic orientation of each grain, as function a macroscopic polarization and crystallographic texture. The method takes advantage of the anisotropy of the properties of the underlying single-crystal phases and delivers a guide to search for material anisotropy |microstructure parameters that are optimal in piezoelectric performance and reliability, and thus establish practical links between structure and macroscopic length scales.     

Effects of thickness on structures and electrical properties of K0.5Na0.5NbO3 thick films derived from polyvinylpyrrolidone-modified chemical solution

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) films with different thicknesses were prepared by polyvinylpyrrolidone (PVP)-modified chemical solution deposition (CSD) method. The KNN films with thickness up to 4.9 μm were obtained by repeating deposition-heating process. All KNN thick films exhibit single perovskite phase and stronger (1 1 0) peak when annealed at 650 °C. The variation of dielectric constant with thickness indicates that there exists a critical thickness for the dielectric constant in the KNN films which should lie in 1.3–2.5 μm. The similar trend is observed for the ferroelectric and piezoelectric properties of KNN films. Both the remnant polarization P_r and the piezoelectric coefficient d₃₃ of KNN thick films increase with the film thickness and become saturated after the critical thickness.     

Lead-free BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 relaxor ferroelectric ceramics for high energy storage

A series of (1-x)(0.65BaTiO₃-0.35Bi_0.5Na_0.5TiO₃)-xNa_0.73Bi_0.09NbO₃ ((1-x)BBNT-xNBN) (x = 0–0.14) ceramics were designed and fabricated using the conventional solid-state sintering method. The microstructure, dielectric property, relaxor behavior and energy storage property were systematically investigated. X-ray diffraction results reveal a pure perovskite structure and dielectric measurements exhibit a relaxor behavior for the (1-x)BBNT-xNBN ceramics. The slim polarization electric field (P-E) loops were observed in the samples with x ≥ 0.02 and the addition of Na_0.73Bi_0.09NbO₃ (NBN) could decrease the remnant polarization (P_r) of the (1-x)BBNT-xNBN ceramics obviously. The sample with x = 0.08 exhibits the highest energy storage density of 1.70 J/cm³ and the energy storage efficiency of 82% at 172 kV/cm owing to its submicron grain size and high relative density. These results show that the (1-x)BBNT-xNBN ceramics may be promising lead-free materials for high energy storage density capacitors.     

Grain size control of lead-free Li0.06(Na0.5K0.5)0.94NbO3 piezoelectric ceramics by Ba and Ti doping

In this study, Ba- and Ti-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ [(1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07)] ceramics were prepared by using conventional solid state reaction method, and the microstructure and electric properties of these samples were investigated. The grain size distribution of non-doped Li_0.06(Na_0.5K_0.5)_0.94NbO₃ ceramics was relatively wide. The microstructure was composed of grains ranging 1.1–5.0 μm in size. However, with increasing Ba and Ti content, the grain size distribution became narrow and the average grain size decreased from 2.0 to 0.9 μm in size. In particular, the microstructure of x = 0.07 sample was composed of grains ranging 0.5–2.2 μm in size. As a result, the frequency dispersion of dielectric constant for the (1 ? x)Li_0.06(Na_0.5K_0.5)_0.94NbO₃–xBaTiO₃ (x = 0–0.07) ceramics was reduced and the mechanical quality factor Q_m was enhanced with increasing Ba and Ti content.     

Phase transition behavior and electrical properties of (1 − x)Bi0.5Na0.5TiO3–x(Na0.53K0.44Li0.04)(Nb0.88Sb0.08Ta0.04)O3 lead-free ceramics

(1 ? x)Bi_0.5Na_0.5TiO₃–x(Na_0.53K_0.44Li_0.04)(Nb_0.88Sb_0.08Ta_0.04)O₃ (BNT–xNKLNST) with x = 0–0.10 lead-free piezoelectric ceramics were prepared by a solid state method, and the structure and electrical properties were investigated in this study. It is found that a morphotropic phase boundary (MPB) of rhombohedral (R) and tetragonal (T) phase exists in the range of 0.03 ≤ x ≤ 0.05 and the structure changes to paraelectric phase when x > 0.07. The samples with x = 0.05 exhibit improved electrical properties owing to the formation of MPB, which are as follows: piezoelectric constant d₃₃ = 120 pC/N, remnant polarization P_r = 39.4 μC/cm² and coercive field E_c = 3.6 kV/mm. These results indicate that the enhanced piezoelectric properties for BNT can be achieved by forming the coexistence of R and T phase.     

Grain-orientated lead-free BNT-based piezoceramics with giant electrostrictive effect

In this study, we propose an effective approach to gain prominent electrostrictive behavior by taking advantage of texture construction in lead-free (Bi_0.5Na_0.5)TiO₃(BNT)-based piezoceramics for actuator applications. The electrostrictive effect was explored systematically in <00 l> textured 0.90(0.83Bi_0.5Na_0.5TiO_3-0.17Bi_0.5K_0.5TiO₃)-0.10NaNbO₃(BNT-BKT-NN) ceramics with respect to grain orientation, temperature and electric fields. It was found that the electrostrctive coefficient Q₃₃ and recoverable strain increase monotonously with the grain orientation increasing. At the optimized microstructure (F_00 l=92%), a giant Q₃₃ with nearly purely electrostrictive characteristics was detected to be around ~0.0354 m⁴C⁻², which was far larger than the reported values of Pb(Mg_1/3Nb_2/3)O₃ (PMN) and the existing BNT-based electrostrictive materials. Meanwhile, the acquired Q₃₃ was found to exhibit excellent stability for the highly textured sample under different temperature or electric field conditions. Our achievement demonstrates that the giant electrostrictive properties with superior temperature and electric field stability for <00 l> textured BNT-BKT-NN ceramic are of great significance in replacing traditional PMN ceramics for actuator applications.     

Crystal structure,microstructure and electrical properties of (1−x−y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 ceramics near MPB prepared via the combustion technique

(1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yBiFeO₃ (BNKFT-x/y with 0.12≤x≤0.24, 0≤y≤0.07) lead-free piezoelectric ceramics have been prepared by the combustion technique. The effects of amounts of x and y on structures and electrical properties were examined. Powders and ceramics can be well calcined and sintered at 750 °C for 2 h and 1025–1050 °C, respectively. The results indicated that the crystalline structure and microstructure changed with the increase of x and y concentrations. XRD results of BNKFT-x/0.03 and BNKFT-0.18/y ceramics with 0.12≤x≤0.24 and 0≤y≤0.07 showed the rhombohedral–tetragonal morphotropic phase boundary (MPB). The addition of y caused a promoted grain growth while the addition of x suppressed the grain growth. The highest density (ρ=5.85 g/cm³), superior dielectric properties at T_c (ε_r=7846 and tan δ=0.02), remnant polarization measured at 40 kV/cm (P_r = 20.1 μC/cm²) and piezoelectric coefficient (d₃₃=213 pC/N) were obtained for x=0.18 and y=0.03.     

Sodium Excess Ta‐Modified (K0.5Na0.5)NbO3 Ceramics Prepared by Reactive Template Grain Growth Method

Lead‐free sodium excess Ta‐modified (K_0.470Na_0.545)(Nb_0.55Ta_0.45)O₃ (KNNT) ceramics were synthesized by a conventional and reactive templated grain growth methods, and their degree of grain orientation, microstructure, dielectric, ferroelectric, and field‐induced strain properties were systematically investigated. A high degree of grain orientation (Lotgering factor F = 80%) was obtained in textured KNNT ceramics. Results showed that textured KNNT ceramics exhibit high grain orientation, dielectric constant, and field‐induced strain as compared to nontextured samples of the same composition. Room temperature unipolar field‐induced strain of K_0.5Na_0.5NbO₃ (KNN) ceramics was enhanced from 0.080% for nontextured sample to 0.115% for textured sample, and their corresponding dynamic piezoelectric coefficients () were improved from 320 pm/V to 460 pm/V, respectively.     

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.     

Microstructural features and electrical properties of copper oxide added potassium sodium niobate ceramics

In this study, effects of 0.5, 1.0 and 1.5 mole% CuO addition on the properties of potassium sodium niobate (K_0.5Na_0.5)NbO₃-KNN ceramics were investigated. Pure KNN and CuO-added KNN pellet samples were sintered at 1100 and 1090 °C for 4 h, respectively. Phase analysis showed that all samples crystallized in pure orthorhombic perovskite phase. Addition of 1.0 and 1.5 mole% CuO caused grain growth, densification and formation of a liquid phase at the grain boundaries. Curie temperature has shifted from 480 to 435 °C with increasing CuO ratio. The most remarkable characteristic of the hysteresis loops were the constricted nature of the 0.5 mole% CuO-added KNN's curve and the antiferroelectric-like appearance of the 1.5 mole% CuO-added KNN's curve. Piezeoelectric properties of d₃₃ = 120 pC/N, k_p = 0.27 and Q_m = 772 were obtained from the 1.5 mole% CuO-added KNN.     

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.     

Piezoelectric and ferroelectric properties of lead-free [Bi1−y(Na1−x−yLix)]0.5BayTiO3 ceramics

Lead-free [Bi_1−y(Na_1−x−yLi_x)]_0.5Ba_yTiO₃ (BNLB-x/y) piezoelectric ceramics were prepared by sintering the constituent oxides, and their piezoelectric and ferroelectric properties studied. The results of X-ray diffraction (XRD) suggest that Li⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ (BNT) lattices to form a solid solution with a single-phase perovskite structure. The ceramics can be well sintered at 1100–1150 °C. The introduction of Li⁺ and Ba²⁺ into Bi_0.5Na_0.5TiO₃ significantly decreases the coercive field, E_c but maintains the large remanent polarization, P_r of the materials. The ceramics exhibit relatively good piezoelectric properties and very strong ferroelectricity: piezoelectric constant, d₃₃ = 208 pC/N, planar electromechanical coupling factor, k_p = 37.0%, remanent polarization, P_r = 38.5 μC/cm², coercive field, E_c = 3.27 kV/mm. The depolarization temperature, T_d of BNLB-0.075/0.04 ceramics is about 190 °C.     

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.