Enhanced piezoelectricity around the tetragonal/orthorhombic morphotropic phase boundary in (Na,K)NbO3–ATiO3 solid solutions

The high density perovskite titanates-modified potassium–sodium niobate solid solutions (1?x)(Na_0.5K_0.5)NbO₃?xBaTiO₃ and (1?x)(Na_0.5K_0.5)NbO₃?xSrTiO₃ have been prepared by the solid state reaction method. The phase diagram summarized from the dielectric property measurement reveals that there exists a tetragonal/orthorhombic morphotropic phase boundary for both the solid solutions. The piezoelectric properties show enhanced behavior at around the tetragonal/orthorhombic morphotropic phase boundary. It is believed that the origin for the high piezoelectric performance (Na,K)NbO₃-based lead-free piezoelectric ceramics is the same as that for the lead zinc niobate-lead titanate solid solution.     

Improved piezoelectric properties of Ag doped 0.94(K0.5–βNa0.5–δ)NbO3–0.06Li1–γNbO3 ceramics by templated grain growth method

In this work, Ag doped 0.94(K_0.5–βNa_0.5–δ)NbO₃–0.06Li_1–γNbO₃ lead-free piezoelectric ceramics have been prepared by templated grain growth (TGG) using cubic (K,Na)NbO₃ single crystal seeds as templates. Specimens were prepared by the conventional mixed method and sintered in controlled atmosphere. TGG method was well known, because that this method can improve piezoelectric properties by increasing the grain size during the sintering process. Sintering caused densification and grain growth of ceramics by the expense of matrix particles. Densification prior to grain growth was found to be necessary to obtain highly textured ceramics. Crystalline properties were analyzed by the XRD method. The effects of TGG on phase structure, microstructure, piezoelectric and dielectric properties of Ag doped 0.94(K_0.5–βNa_0.5–δ)NbO₃–0.06Li_1–γNbO₃ ceramics were investigated.     

Structure and piezoelectric properties of K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript> based lead-free piezoceramics with slight deviation from A-site K or Na stoichiometry

The dependence of structure and piezoelectric response d₃₃ on individual excess A-site K or Na in K_0.5Na_0.5NbO₃ based lead-free piezoceramics is reported. The coexistence of orthorhombic, tetragonal, and monoclinic phases at room temperature is observed in the investigated 0.94K_0.5+xNa_0.5NbO₃–0.06LiNbO₃ and 0.94K_0.5Na_0.5+yNbO₃–0.06LiNbO₃ ceramics (x, y = 0–0.025). The weight ratio of three phases, orthorhombic-tetragonal phase transition temperature, and d₃₃ value are highly sensitive to A-site K or Na nonstoichiometry. A-site Na nonstiochiometry plays a dominant role in enhancing the electrical properties of KNN by significantly reducing the weight ratio percentage of monoclinic phase and shifting the orthorhombic-tetragonal phase transition temperature to lower temperatures.     

Effect of sintering temperature on piezoelectric and dielectric properties of 0.98(Na0.5K0.5)NbO3-0.02Li(Sb0.17Ta0.83)O3+0.01wt%ZnO ceramics

We studied sintering temperature to enhance the piezoelectric and dielectric properties of 0.98(Na_0.5?K_0.5)NbO₃-0.02Li(Sb_0.17Ta_0.83)O₃?+?0.01wt%ZnO (hereafter NKN-LST+ZnO) lead free piezoelectric ceramics. The synthesis and sintering method were the conventional ceramic technique and sintering was executed at 1080?～?1120°C. We found that optimal sintering temperature and NKN-LST+ZnO ceramics showed the highest piezoelectric properties and dielectric properties at the optimal sintering temperature. The NKN-LST+ZnO ceramics sintered at 1090°C show a superior performance with piezoelectric constant d ₃₃?=?185 pC/N, k _p?=?0.36, ε ₃₃ ^T /ε₀?=?491 respectively. These results reveal that NKN-LST+ZnO ceramics are promising candidate materials for lead-free piezoelectric application.     

Dielectric,ferroelectric and bipolar electric field induced strain properties of MPB composition of NBT-xKNN system

(1-x) (Na_0.5Bi_0.5TiO₃)-xK_0.5Na_0.5NbO₃/NBT-xKNN [x?=?0.07, 0.06, 0.05] ferroelectric ceramics were prepared by solid state synthesis route (SSSR). The effects of KNN contents on the microstructure, dielectric, piezoelectric and ferroelectric properties of the NBT-xKNN system were investigated in detail. For single perovskite phase formation, the calcination temperature was optimized at 800 °C for 6 h. From the XRD study, the morphotropic phase boundary (MPB) was confirmed for x?=?0.07 composition. For better densification, the sintering temperature was optimized for 1150 °C for 4 h. SEM micrographs illustrate the closely packed and non-uniform distribution of grains. Diffusive type of behaviour was observed in all the ceramics. Polarization (P) vs. electric field (E) study confirmed the ferroelectric nature of the NBT-xKNN ceramics. The bipolar field-induced strain measurement for all the ceramic samples showed butterfly-shaped loops indicating their piezoelectric nature. Among all the different compositions in MPB region, high dielectric constant (ε_r) of?~?3011, high remnant polarization (P _r ) of 17.88μC/cm² and high strain % of 0.41, were obtained in NBT-xKNN system with x?=?0.07 confirming the existence of MPB at this composition.     

Dielectric and piezoelectric properties of sodium lithium niobate Na1−x Li x NbO3 lead free ferroelectric ceramics

High density sodium lithium niobate lead free ceramics near the morphtropic phase boundary [Na _x Li_1?x NbO₃, (LNN), x?=?0.12] were prepared by the solid state reaction method. XRD patterns showed that the lattice structures were changed after polarization. The temperature dependence of the dielectric constant and dielectric loss, pyroelectric coefficient and DSC curves of LNN ceramics showed that there exist three phase transitions from room temperature up to the Curie temperature. The hysteresis loop and piezoelectric properties were measured and discussed.     

Lead-free K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>–Bi<Subscript>0.5</Subscript>Li<Subscript>0.5</Subscript>ZrO<Subscript>3</Subscript>–BiAlO<Subscript>3</Subscript> ternary ceramics: Structure and piezoelectric properties

Lead-free perovskite (0.995–x)(K_0.5Na_0.5)NbO₃–x(Bi_0.5Li_0.5)ZrO₃–0.005BiAlO₃ ternary piezoelectric ceramics were projected and prepared by a conventional solid-state method. A research was conducted on the effects of (Bi_0.5Li_0.5)ZrO₃ content on the structure and piezoelectric properties of the ceramics. By combining the X-ray diffraction patterns with the temperature dependence of dielectric properties, a rhombohedral–orthorhombic–tetragonal phase coexistence was identified for the ceramics with 0.02 ≤ x ≤ 0.025, and a rhombohedral–tetragonal phase boundary was determined in the composition x = 0.03. Upon further increasing the (Bi_0.5Li_0.5)ZrO₃ content, the rhombohedral–tetragonal phase boundary transformed to a single rhombohedral structure with x ≥ 0.035. An obviously improved piezoelectric activity was obtained for the ceramics with compositions in and around the rhombohedral–tetragonal phase boundary, among which the composition x = 0.025 exhibited the maximum values of piezoelectric constant d ₃₃, and planar and thickness electromechanical coupling coefficients (k _p and k _t), of 252 pC/N, 0.366, and 0.466, respectively. In addition, the ceramic with x = 0.025 was found to possess a relatively high Curie temperature of 368 °C, suggesting it may have a prospect for applications at elevated ambient temperatures.     

Synthesis and piezoelectric properties of (1 − x)(Na0.5K0.5)NbO3–x(Ba0.95Sr0.05)TiO3 ceramics

(1 ? x)(Na_0.5K_0.5)NbO₃–x(Ba_0.95Sr_0.05)TiO₃ [(1 ? x)NKN–xBST] ceramics were synthesized by the conventional solid-state sintering, and their microstructure and piezoelectric properties were investigated. The sintering temperature of the specimens was 1075 °C in air atmosphere and a morphotropic phase boundary (MPB) was observed in the specimens with 0.03 ≤ x ≤ 0.05. Compared with the piezoelectric properties of the NKN ceramics, the enhanced d ₃₃ value of 136 pC/N and ? ₃ ^T /? _o value of 671 were obtained for the (1 ? x)NKN–xBST specimens with x = 0.03.     

Effect of BiAlO3 concentration on the dielectric and piezoelectric properties of lead-free (Bi0.5Na0.5)0.94Ba0.06TiO3 piezoelectric ceramics

In this paper, lead-free (1-x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-xBiAlO₃ (BNBT-BA, x?=?0, 0.010, 0.015, 0.020, 0.025, and 0.030) piezoelectric ceramics were synthesized using a conventional solid-state reaction method. The effect of BiAlO₃ concentration on dielectric, ferroelectric and piezoelectric properties were investigated. The ferroelectric and piezoelectric properties of BNBT ceramics are significantly influenced by the presence of BA. In the composition range studied, X-ray diffraction revealed a perovskite phase with the coexistence of rhombohedral and tetragonal phases. The temperature dependence of dielectric properties showed that the depolarization temperature (T _d) shifted towards lower temperatures and that the degree of diffuseness of the phase transition around T _d and T _m became more obvious with increasing BiAlO₃ content. The remanent polarization increased with increasing BA, and reached a maximum value of 30 μC/cm² at x?=?0.020. As a result, at x?=?0.020, the piezoelectric constant (d ₃₃) and the electromechanical coupling factor (k _p) of the ceramics attained maximum values of 188 pC/N and 34.4 %, respectively. These results indicate that BNBT-BA ceramics is a promising candidate for lead-free piezoelectric materials.     

Lead-free Na0.5K0.5NbO3 piezoelectric ceramics fabricated by spark plasma sintering: Annealing effect on electrical properties

Niobate ceramics such as NaNbO₃ and KNbO₃ have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na_0.5K_0.5NbO₃ ceramics with submicron grains were prepared using SPS, whose density was raised to 4.47 g/cm³ (>99% of the theoretical density) at 920 °C. Reasonably good ferroelectric and piezoelectric properties were obtained in the SPSed Na_0.5K_0.5NbO₃ ceramics after annealing in air. The effect of annealing time on the electrical properties was investigated to determine optimal processing condition. The piezoelectric parameter (d ₃₃) of the Na_0.5K_0.5NbO₃ ceramics annealed properly reached 148 pC/N.     

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr _x Ti_1-x )O₃ (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO₃ and (Bi_1/2Na_1/2)TiO₃-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi_1/2Na_1/2)TiO₃-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.     

Influences of ScTa co-substitution on the properties of Ultra-high temperature Bi3TiNbO9-based piezoelectric ceramics

The effect of (Sc,Ta) doping on the properties of Bi₃TiNbO₉-based ceramics was investigated. The (Sc,Ta) modification greatly improves the piezoelectric activity of Bi₃TiNbO₉-based ceramics and significantly decreases the dielectric dissipation. The d₃₃ of Bi₃(Ti_0.96Sc_0.02Ta_0.02)NbO₉ was found to be 12 pC/N, the highest value among the Bi₃TiNbO₉-based ceramics and almost 2 times as much as the reported d₃₃ values of the pure BTNO ceramics (~6pC/N). The high T_C (higher than 900 °C) and stable piezoelectric and dielectric properties, demonstrating that the (Sc,Ta) modified Bi₃Ti_1?xSc_x/2Ta_x/2NbO₉-based material a candidate for ultrahigh temperature applications. The new (ScTa) modification has an important typical significance, the way should be used for reference in constructing the new high performance materials.     

Temperature dependent electrical properties of 0.95[(K0.5Na0.5)(1-x)Ag x NbO3]-0.05LiSbO3 ceramics

Lead free 0.95[(K_0.5Na_0.5)_1-x Ag _x NbO₃]-0.05LiSbO₃ (KNAN-LS) ceramics with x?=?0.02, 0.04, 0.06 and 0.08 have been synthesized by conventional solid state reaction route (CSSR). XRD analysis confirmed the presence of a mixed structure for x?=?0.06. The orthorhombic?Ctetragonal polymorphic phase transition (PPT) temperature and the Curie temperature (T_c) decreased with the increase in Ag⁺ ion content in KNAN-LS ceramics. The relationship between the PPT of the ceramics and the temperature dependence of electrical properties of KNAN-LS ceramics were discussed in detail. The KNAN-LS ceramics with x?=?0.06 showed better piezoelectric and electromechanical properties (d₃₃?=?227pC/N and k_p?=?42.5?%).     

Effect of heating rate on the structure evolution of (K0.5Na0.5)NbO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃–xLiNbO₃ (abbreviated an KNLN) have been synthesized by traditional ceramics process. Effects of heating rate on the phase structure, microstructure evolution and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃–xLiNbO₃ were investigated. Results show that the heating rate has no effects on the phase structures. However, the fracture surface of the 0.94(K_0.5Na_0.5)NbO₃ −0.06 LiNbO₃ ceramics transforms from intergranular fracture mode to a typical transgranular fracture mode with the increasing of the heating rate. This result is ascribed to the presence of agglomerates of grains which exhibit different sintering behavior at diverse heating rates. The 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramic sintered at 1080°C with heating rate of 5°C/min shows the optimum piezoelectric properties(d ₃₃ = 210 pC/N, k _p = 0.403 and k _t = 0.498).     

Effects of co-doped CaO/MnO on the piezoelectric/dielectric properties and phase transition of lead-Free (Bi0.5Na0.5)0.94Ba0.06TiO3 piezoelectric ceramics

The piezoelectric properties of (1?x)(Bi_0.5Na_0.5)TiO₃-xBaTiO₃ ceramics were reported and their piezoelectric properties reach extreme values near the MPB (about x?=?0.06). The X-ray analysis of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics for all compositions exhibited a pure perovskite structure without any secondary phase. Within a certain ratio of contents, the co-doped ceramics enhanced piezoelectric coefficient (d ₃₃ ), lowered the dielectric loss, and increased the sintered density. The temperature dependence of relative dielectric permittivity (K ₃₃ ^T ) reveals that the solid solutions experience two phase transitions, ferroelectric to anti-ferroelectric and anti-ferroelectric to relaxor ferroelectric, which can be proven by P-E hysteresis loops at different temperatures. In addition, the specimen containing 0.04/0.01 wt.% CaO/MnO showed that the coercive field E _c was a minimum value of 26.7 kV/cm, while the remnant polarization P _r was a maximum value of 38.7 μC/cm², corresponding to the enhancement of piezoelectric constant d₃₃ of 179 pC/N, electromechanical coupling factor k _p of 37.3%, and relative dielectric permittivity K ₃₃ ^T of 1137. (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics co-doped with CaO/MnO were considered to be a new and promising candidate for lead-free piezoelectric ceramics owing to their excellent piezoelectric/dielectric properties, which are superior to an un-doped BNBT system.     

Investigation of the BaTiO3–BaMg1/3Nb2/3O3 system: Structural, dielectric, ferroelectric and electromechanical studies

Lead-free (1?x) BaTiO₃ –x BaMg_1/3Nb_2/3O₃ ceramics with x?=?0.03, 0.04, 0.05 and 0.06 were prepared by solid-state synthesis. The effects of the Ba(Mg_1/3Nb_2/3)O₃ addition on the phase composition, dielectric and ferroelectric properties, as well as the electromechanical response of the classic ferroelectric BaTiO₃ were investigated. The room-temperature X-ray diffraction analyses of all the ceramics revealed a perovskite phase after sintering at 1300 °C with a composition-dependent symmetry. The samples with a lower concentration of Ba(Mg_1/3Nb_2/3)O₃, i.e., x?=?0.03 and 0.04, were tetragonal, while the samples with x?=?0.05 and 0.06 were found to be cubic. The result is in agreement with the dielectric, ferroelectric and electromechanical properties. With x increasing from 0.03 to 0.06 the temperature of the diffused maximum of the dielectric permittivity decreased from 348 K to 265 K. All the ceramics showed a large electromechanical response: the calculated room-temperature electrostrictive coefficient M ₃₃ of the sample with x?=?0.06 was 1.4 · 10^?16?m²/V², which is comparable to the value measured for Pb(Mg_1/3Nb_2/3)O₃ ceramics.     

Dielectric properties and defect mechanisms of (1-<Emphasis Type="Italic">x</Emphasis>)Ba(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> -<Emphasis Type="Italic">x</Emphasis>BiYbO<Subscript>3</Subscript> ceramics

(1-x)Ba(Fe_0.5Nb_0.5)O₃ -xBiYbO₃ (BFN-xBY) ceramics were prepared by a conventional solid-state reaction method. The dielectric properties and relaxation behavior of BFN-xBY ceramics were analyzed according to dielectric and impedance spectroscopy. Dielectric permittivity of the ceramics increases with increasing temperature below 500 K then remains unchanged up to 700 K, while corresponding loss factor decreases with the increase of temperature below 500 K then increase slowly. Defect compensation mechanism of this system was analyzed in detail. The giant dielectric behavior of the ceramics arises from the internal barrier layer capacitor (IBLC) effect. Polarization effect at insulating grain boundaries between semiconducting grains accompanied by a strong Maxwell-Wagner (MW) relaxation mode. The characteristic of grain boundaries was revealed using impedance spectroscope and the universal dielectric response law.     

Relaxor behavior of piezoelectric Pb(Yb1/2Nb1/2)O3-PbTiO3 ceramics sintered at low temperature

A (100-x)Pb(Yb_1/2Nb_1/2)O₃-xPbTiO₃ [PYN-PTx] solid solution system with 49.0????×????51.0 was prepared using a conventional ceramics process and sintered at low temperature. When excess PbO was added into the PYN-PTx system, all samples were sintered at temperatures as low as 800°C with good dielectric and piezoelectric properties. It is suggested that a liquid phase with excess PbO was formed during the sintering and improved the densification of PYN-PTx ceramics at low temperatures. For the PYN-PTx binary system, it was found that the temperature dependence of the relative permittivity follows a Curie?CWeiss Law above the deviation temperature (T_D) at high temperatures. Good piezoelectric properties of d ₃₃ ?=?510 pC/N, ?? _r ?=?2800 at RT, k _p?=?0.57, and k _t?=?0.42 with T_c?=?373°C were obtained for PYN-PT49.5 ceramics sintered at 800°C for 8 h.     

Effect of sintering temperature on microstructure and piezoelectric properties of Pb-free BiFeO3–BaTiO3 ceramics in the composition range of large BiFeO3 concentrations

Lead-free (1-x)BiFeO₃–xBaTiO₃ [(1-x)BF-xBT] piezoelectric ceramics in the range of large BF concentrations were prepared by conventional oxide-mixed method at various sintering temperatures. The sintering temperatures have a significant effect on the microstructure of the ceramics, and the composition has a remarkable effect on optimal sintering temperature of the ceramics, which are closely related with piezoelectric properties. The grain size increased with increasing sintering temperature and the optimal sintering temperature increased with increasing BT content. The ceramics with x?=?0.275 sintered at 990 °C exhibit enhanced electrical properties of d ₃₃?=?136pC/N and k _p?=?0.312 due to the polarization rotation mechanisms at MPB and desired microstructure. These results show that the ceramic with x?=?0.275 is a promising lead-free high-temperature piezoelectric material.     

Pyroelectric, dielectric, and piezoelectric properties of MnO2-doped (Na0.82 K0.18)0.5Bi0.5TiO3 lead-free ceramics

MnO₂ doped (Na_0.82 K_0.18)_0.5Bi_0.5TiO₃ lead-free piezoelectric ceramics were prepared by conventional solid-state reaction process and the effect of MnO₂ addition on the pyroelectric, piezoelectric and dielectric properties were studied. The experiment results showed that the pyroelectric, piezoelectric, and dielectric properties strongly depended on MnO₂ addition in the (Na_0.82 K_0.18)_0.5Bi_0.5TiO₃ ceramics. Excellent electrical properties were obtained in (Na_0.82 K_0.18)_0.5Bi_0.5TiO₃ with 0.8?mol% MnO₂. The large dielectric loss of pure BNT ceramics was significantly reduced, the piezoelectric constant was improved, and it also showed excellent pyroelectric properties when compared with other lead free ceramics, with pyroelectric coefficient p?=?17?×?10^?4?C/m²K and figure of merit F _d ?=?6.56?×?10^?5?Pa^?0.5. With these outstanding pyroelectric properties, the 0.8?mol% MnO₂ doped (Na_0.82 K_0.18)_0.5Bi_0.5TiO₃ ceramic can be a promising material for pyroelectric sensor applications in future.