Piezoelectric K0.5Na0.5NbO3 Ceramics Textured Using Needlelike K0.5Na0.5NbO3 Templates

Improved performance by texturing has become attractive in the field of lead‐free ferroelectrics, but the effect depends heavily on the degree of texture, type of preferred orientation, and whether the material is a rotator or extender ferroelectric. Here, we report on successful texturing of K_0.5Na_0.5NbO₃ (KNN) ceramics by alignment of needlelike KNN templates in a matrix of KNN powder using tape casting. Homotemplated grain growth of the needles was confirmed during sintering, resulting in a high degree of texture parallel to the tape casting direction (TCD) and the aligned needles. The texture significantly improved the piezoelectric response parallel to the tape cast direction, corresponding to the direction of the strongest <001>_pc orientation, while the response normal to the tape cast plane was lower than for a nontextured KNN. In situ X‐ray diffraction during electric field application revealed that non‐180° domain reorientation was enhanced by an order of magnitude in the TCD, compared to the direction normal to the tape cast plane and in the nontextured ceramic. The effect of texture in KNN is discussed with respect to possible rotator ferroelectric properties of KNN.     

Screening Sintering Aids for (K0.5Na0.5)NbO3 Ceramics

Searching for suitable sintering aids for ceramic materials is important and tedious work. In this study, we introduce a simple and effective method, named liquid phase screening method (LPSM), for rapidly screening sintering aids for KNN ceramics. By measuring the structure and properties, we demonstrated that the suitable sintering aids for KNN can be quickly determined by LPSM. The new sintering aids found by this method, GeO₂ and borax which have not been reported before, lead to improved properties.     

K0.5Na0.5NbO3-LiSbO3-BiFeO3无铅压电陶瓷的烧结特性

采用传统无压固相烧结法制备0.996(0.95K0.5NbO3-0.05LiSbO3)-0.004BiFeO3[0.996(0.95KNN-0.05LS)-0.004BF]无铅压电陶瓷,着重研究烧结保温时间对陶瓷结构、压电性能与介电性能和Curie温度Tc的影响.结果表明:随着烧结保温时间的延长,陶瓷趋于形成更稳定的四...     

Effects of GeO2 Addition on Sintering and Properties of (K0.5Na0.5)NbO3 Ceramics

Lead‐free (K_0.5Na_0.5)NbO₃ (KNN) piezoelectric ceramics doped with different amounts of GeO₂ were prepared and characterized. GeO₂ was found to effectively improve the sinterability and piezoelectric properties of the material. The improvement in the sinterability is ascribed to the formation of a liquid phase, which decreased the sintering temperature from 1080°C to 1010°C. The improvement in the properties is attributed to the replacement of Nb⁵⁺ with Ge⁴⁺ to form acceptor dopants. The following optimized properties were obtained from the KNN ceramic with 0.75 wt% GeO₂: piezoelectric constant (d₃₃) = 126 pC/N, planar electromechanical coupling coefficient (k_p) = 42.8%, mechanical quality factor (Q_m) = 140, and dielectric loss (tanδ) = 3.8%.     

(1-x)KNN-xNKBT无铅压电陶瓷的结构及其电性能研究

采用传统固相法制备了(1-x)K0.5Na0.5NbO3-x(Na0.8K0.2)0.5Bi0.5TiO3(x=0-5%)无铅压电陶瓷,研究了(Na0.8K0.2)0.5Bi0.5TiO3的不同引入量对其物相结构、显微形貌、介电性能以及压电性能的影响。结果表明:所有样品都具有钙钛矿结构;随着x的增加,室温下样品从正交相逐渐向四方相过渡并且居里温度向低温方向移动,样品的压电常数d33与机电耦合系数kp均先升高后降低。该体系多晶型转变PPT位于2%≤x≤3%,当x=3%时,样品的压电性能达到最佳,其中:d33=189pC/N,kp=41%,Qm=96,tanδ=0.028。     

K0.5Na0.5NbO3-BaCu0.5W0.5O3无铅压电陶瓷的结构与性能

以传统固相法工艺制备(1-x)K0.5Na0.5NbO3-xBaCu0.5W0.5O3[(1-x)KNN-xBCW]无铅压电陶瓷,研究不同BCW掺量(x=0％,0.1％,0.25％,0.5％,1.0％,摩尔分数,下同)对KNN陶瓷的晶体结构和电性能的影响,结果表明:x＜0.5％时,KNN陶瓷的相结构没有改变,仍为正交相...     

High‐Performance (Na0.5K0.5)NbO3 Thin Film Piezoelectric Energy Harvester

The pseudocubic structure of a (Na_0.5K_0.5)NbO₃ (NKN) film grown on a Pt/Ti/SiO₂/Si substrate changed to an orthorhombic structure when the film was transferred onto a polyimide substrate. Piezoelectric constant for the transferred NKN film increased considerably from 74 ± 11 to 120 ± 18 pm/V because the crystal structure of the film had changed from pseudocubic to orthorhombic. A gold interdigitated electrode was deposited onto the transferred NKN film to synthesize a NKN piezoelectric energy harvester. The NKN piezoelectric energy harvester was poled before bending under a 100 kV/cm DC electric field across the electrodes. When a strain of 0.85% and a strain rate of 4.05%/s were applied to the NKN piezoelectric energy harvester, it produced a maximum output voltage of 1.9 V and a current of 38 nA, corresponding to a power density of 2.89 μW/cm³.     

Piezoelectric enhancements in K0.5Na0.5NbO3-based ceramics via structural evolutions

The current work aims to compare the effect of systematic A-site and B-site substitutions on the piezoelectricity of Ka_0.5Na_0.5NbO₃ (KNN)-based perovskite ceramics. The A-site elements was replaced by Li⁺ while Nb5⁺ was substituted by Sb⁵⁺ to form (K_0.4675Na_0.4675Li_0.065)NbO₃ (KNLN) and (K_0.4675Na_0.4675Li_0.065)(Nb_0.96Sb_0.04)O₃ (KNLNS) respectively. The ceramics were prepared using solid-state sintering method. The density of the ceramics steadily improved with the substitutions while the crystal structure evolved from monoclinic (in KNN) to the coexistence of monoclinic and tetragonal (in KNLN) and finally tetragonal in KNLNS. Distinct variations on size and morphology were recorded. Although density, crystal structure and morphology have minor effect on the E_c, they imposed considerable influences on P_r, d₃₃ and k_p. Despite relatively lower density, KNLN exhibited the highest P_r, d₃₃ and k_p at 9.80 μC/cm²,185 pC/N and 0.43 respectively signifying the positive enhancement brought by the co-existence of monoclinic and tetragonal crystal structures. More importantly, this work systematically proved that the co-existence of both structures signified the morphotropic phase boundary (MPB) composition as the primary factor for the enhancement of KNN piezoelectric properties.     

Preparation and characterization of (K0.5Na0.5)NbO3 ceramics

In this paper the preparation and characterization of the ceramic material (K_0.5Na_0.5)NbO₃ (KNN) has been studied. Although conventional processing of KNN is often reported to result in sintered bodies lacking sufficient density, samples produced in this work exhibit theoretical density over 95% and yield superior piezoelectric properties than those obtained by the same method and reported previously. The electromechanical coupling coefficient in the thickness direction, k_t, is found to reach 45%. Apart from k_t, the piezoelectric coefficients in longitudinal and planar directions (d₃₃ of 100pC/N and d₃₁ of 43pC/N), hysteresis loop, pyroelectric coefficient measurements and dielectric properties are presented.     

聚合物前驱体法低温制备铌酸钾钠超细粉体

以柠檬酸为配位剂与金属离子配合,水作为溶剂,乙二醇为酯化剂,通过聚合物前驱体法制备(K0.5Na0.5)NbO3粉体.研究了柠檬酸(CA)和乙二醇(EG)的用量和pH值对溶胶性能的影响.采用XRD和TG-DSC研究了(K0.5Na0.5)NbO3晶相的形成过程,用SEM对所制得粉体的表面形貌进行了分析.结果表明:当前驱体溶胶的pH值为7.5,n(CA):n(metalion)=3:1,n(CA):n(EC)=1:2时,可以获得稳定性好的溶胶.凝胶前驱体在加热过程中先反应生成NaNbO3和K2Nb8O21,然后二者固相反应形成K0.5Na0.5NbO3.大部分晶粒呈方形的粒状,少数颗粒呈长柱状.讨论了晶粒形貌的形成原因.     

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B₂O₃–Bi₂O₃—CdO. It exhibited the following dielectric and piezoelectric properties: K_p= 0.52 to 0.58, Q_m= 1000, ε^T₃₃/ε₀= 800 to 1000, tan δ= 50 × 10⁻⁴, ρ= 7.56 to 7.64 g/cm³. Ceramics with the aid of suitable dopants (CdO, SiO₂, and excess PbO) in the Pb-(Ni_1/3Nb_2/3)O₃—PZT family could be sintered at 860° to 900°C. For these materials, K_p= 0.56 to 0.61, Q_m= 1000, ε^T₃₃/ε₀= 1500 to 2000, tan δ≤ 50 × 10⁻⁴, ρ= 7.80 to 8.03 g/cm³. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.     

Electric Field Cycling Induced Large Electrostrain in Aged (K0.5Na0.5)NbO3–Cu Lead‐Free Piezoelectric Ceramics

In this work, we studied aging and fatigue behaviors of 1.0 mol%Cu‐modified (Na_0.5K_0.5)NbO₃ lead‐free ceramics with double‐loop‐like characteristic, and found an interesting result of electric field cycling induced large electrostrain in the aged samples. After the “aging” treatment, the hysteresis loop “closes” totally and constricts completely at zero electric field, accompanying with a large nonlinear electrostrain of about 0.26%. More interestingly, the electrostrain in the aged sample is sensitive to the field cycling, which is pushed to a higher level of 0.47% approaching that of some Pb‐based antiferroelectric materials. A microscopic model of the domain switching‐related electrostrain effect is proposed to explain the field cycling induced large electrostrain in the aged samples. It can be identified due to the weakening of intrinsic restoring force for reversing the switched domains, induced by the point defects (defect dipoles) migration aligning along field direction during cycling, evident by the gradually “opened” hysteresis loop with cycling.     

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%.     

Enhanced Piezoelectric Properties and Thermal Stability in the (K0.5Na0.5)NbO3:ZnO Lead‐Free Piezoelectric Composites

The piezoelectric properties of (K_0.5Na_0.5)NbO₃ (KNN) are normally enhanced by chemical substitutions or doping to form solid solutions. In this study, we report that the piezoelectric properties of KNN and thermal stability of piezoelectric coefficient d₃₃ can be both enhanced by forming the composite of KNN:ZnO. The d₃₃ of KNN:0.2ZnO can be improved to 110 pC/N by introducing the ZnO nanoparticles, which is better than the pure KNN (d₃₃ = 85 pC/N). The Curie temperature (T_C = 407°C) remains well comparable to the pure KNN (T_C = 408°C). Furthermore, the thermal stability of both remanent polarization (P_r) and piezoelectric parameter (d₃₃) is improved. The enhanced thermal stability could be related to the induced built‐in electric field or the enhanced sinterability by the addition of ZnO. The present results may help to optimize the piezoelectric properties of lead‐free materials by forming composite.     

Optical and Tunable Dielectric Properties of K0.5Na0.5NbO3–SrTiO3 Ceramics

Pure perovskite K_0.5Na_0.5NbO₃–xSrTiO₃ (x = 0.16, 0.17, 0.18, and 0.19) ceramics were prepared by using a solid‐state reaction process. The ceramics were optically transparent for visible and near‐infrared wavelengths. Then, high tunability (24.1%) and low dielectric loss (0.016) for the x = 0.18 sample indicated the transparent ceramics could be used in tunable devices. The Lorentz‐type relation fitting for the temperature dependence of dielectric permittivity showed that these ceramics had a typical relaxor behavior, and the polar nanoregions were related to the tunable dielectric properties. The nonlinear dielectric behavior was further explored by the Johnson model combined with Langevin terms, which revealed that the polar nanoregions contributed to the nonlinear ε(E) dependencies with contributions of 12.3%, 11.6%, 5.9%, and 3.6% for x = 0.16, 0.17, 0.18, and 0.19, respectively.     

Phase-composition dependent domain responses in (K0.5Na0.5)NbO3-based piezoceramics

Lead-free (K_0.5Na_0.5)NbO₃-based (KNN) piezoceramics featuring a polymorphic phase boundary (PPB) between the orthorhombic and tetragonal phases at room temperature are reported to possess high piezoelectric properties but with inferior cycling stability, while the ceramics with a single tetragonal phase show improved cycling stability but with lower piezoelectric coefficients. In this work, electric biasing in-situ transmission electron microscopy (TEM) study is conducted on two KNN-based compositions, which are respectively at and off PPB. Our observations reveal the distinctive domain responses in these two ceramics under cyclic fields. The higher domain wall density in the poled KNN at PPB contributes to the high piezoelectric properties. Upon cycling, however, a new microstructure feature, “domain intersection”, is directly observed in this PPB composition. In comparison, the off-PPB KNN ceramic develops large domains during poling, which experience much less extent of disruption during cycling. Our comparative study provides the basis for understanding the relation between phase composition and piezoelectric performance.     

Photoluminescence Properties of Er/Pr‐Doped K0.5Na0.5NbO3 Ferroelectric Ceramics

Er/Pr‐doped K_0.5Na_0.5NbO₃ ceramics have been fabricated and the effects of Pr³⁺ on their photoluminescence properties have been investigated systematically. The visible upconversion emissions, near‐infrared and mid‐infrared downconversion emissions of Er³⁺ ions under the excitation of 980 nm have been studied in detail. The effects of Pr³⁺ on PL properties and energy‐transfer processes have also been elucidated. By selecting an appropriate excitation source, simultaneous visible downconversion emissions of Er³⁺ and Pr³⁺ ions can be realized, and the emission colors of the ceramics can be tuned via the concentration of Pr³⁺ ions in a wide range from yellowish green to yellow. Our results also reveal that the photoluminescence emissions of the ceramics can be enhanced by the alignment of polarization of the ferroelectric host.     

Crystal growth and ferroelectric property of Na0.5K0.5NbO3 and Mn-doped Na0.5K0.5NbO3 crystals grown by floating zone method

Na_0.5K_0.5NbO₃ (NKN) and Mn-doped NKN crystals, which are one of the promising candidates of lead-free piezoelectric materials, were grown by using a floating zone (FZ) method. The resulting crystal growth was compared with crystal growth that resulted from using a flux method in a previous study. In the crystal grown by FZ method under where the growth rate was controlled to 3 mm/h, thin layers formed parallel and perpendicular directions to the growth direction. In the crystal grown by FZ method, the crystal structure could not be classified as having the orthorhombic lattice of Amm2, which was observed in the crystal grown using a flux method. It was found that doped Mn was substituted in the perovskite-type lattice of NKN. Pure NKN crystals showed 90° domains that had a zig-zag shape, whereas Mn-doped NKN crystals were aligned to the domain layers in straight lines. It was confirmed that Mn-doped NKN crystal showed a square P–E hysteresis loop.     

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.     

Impedance Spectroscopy of (Bi1/2Na1/2)TiO3–BaTiO3 Ceramics Modified with (K0.5Na0.5)NbO3

The electrical and dielectric properties of (1 ? x)(0.94Bi_1/2Na_1/2TiO₃–0.06BaTiO₃)–x(K_0.5Na_0.5NbO₃) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n‐type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature‐dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN.