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.     

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.     

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.     

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.     

Low-Temperature Sintering of (K0.5Na0.5)NbO3 Piezoelectric Ceramics

(K_0.5Na_0.5)NbO₃ piezoelectric ceramics can be sintered at a temperature as low as 750 °C for 5 h by incorporating Li₂CO₃ + Bi₂O₃ + ZnO as the sintering aid, whereas the conventional sintering temperature is around 1,100 °C. The optimal “soft” piezoelectric properties are obtained for ceramics sintered at 850 °C for 5 h. The dielectric permittivity (ε), piezoelectric coefficient (d ₃₃), electromechanical coupling (k _p) and mechanical quality factors (Q _m) of (K, Na)NbO₃ modified with 5.5 wt% sintering aids are 1,436, 90 pC/N, 0.3 and 10, respectively. These values are similar to the values obtained for (K_0.5Na_0.5)NbO₃ ceramics sintered above 1,100 °C. The underlying mechanism for abrupt change of dielectric permittivity is explained.     

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.     

New Lead‐Free (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)ZrO3 Ceramics with High Piezoelectricity

For enhancing the piezoelectric properties of ceramics (Bi_0.5Na_0.5)ZrO₃ (BNZ) was used to partially substitute (K_0.5Na_0.5)NbO₃ (KNN). The addition of BNZ changes the symmetry of KNN ceramics from orthorhombic to tetragonal, and finally to rhombohedral phase. A new phase boundary with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature is identified for KNN–0.050BNZ ceramics, where optimum electrical properties were obtained: d₃₃ = 360 pC/N, k_p = 32.1%, ε_r = 1429, tanδ = 3.5%, and T_C = 329°C. The results indicated a new method for designing high‐performance lead‐free piezoelectric materials.     

Excellent Transmittance Induced Phase Transition and Grain Size Modulation in Lead‐Free (1–x)(K0.5Na0.5)NbO3–xLaBiO3 Ceramics

A novel lead‐free excellent transmittance electro‐optic ceramics (1–x)(K_0.5Na_0.5)NbO₃–xLaBiO₃ (KNN‐LB, x = 0.000, 0.005, 0.010, 0.015, 0.020, 0.025, 0.040, 0.060) were fabricated by traditional pressureless ceramics processing procedure. The effects of LaBiO₃ dopant concentration x on the microstructure, phase transition, optical property, and electrical properties were studied systematically. The X‐ray diffraction results indicated that the KNN‐LB ceramics with x ≥ 0.025 have the pseudocubic phase. The morphology, density, and microstructure of the KNN‐LB ceramics were characterized by scanning electronic microscopy and optical microscopy. In particular, the KNN‐LB ceramics (0.05 mm thickness) with x = 0.025 exhibited the highest transmittance of 74.00% in the visible spectrum comparable to the 72.00% transmittance of the lead lanthanum zirconate titanate (PLZT 9/65/35 of 0.127 mm thickness). In addition, the related mechanism of transparency variation induced by phase transition and grain size modulation were discussed thoroughly. Finally, the dielectric and ferroelectric properties of as‐prepared KNN‐LB ceramics were also investigated to further clarify the relationship between transparency and relaxor behavior.     

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.     

The NbO6 octahedral distortion and phase structural transition of Eu3+‐doped K0.5Na0.5NbO3–xLiNbO3 ferroelectric ceramics

A small quantity of Eu³⁺ ions were doped in the lead‐free ferroelectric K_0.5Na_0.5NbO₃–xLiNbO₃ (KNN–xLN, 0 ≤ x ≤ 0.08) ceramics to investigate the NbO₆ octahedral distortion induced by the increasing LN content. In addition, the phase structure, ferroelectric, and photoluminescence properties of K_0.5Na_0.5NbO₃–xLiNbO₃:0.006Eu³⁺ (KNN–xLN:0.006Eu³⁺) lead‐free piezoelectric ceramics were characterized. All the X‐ray diffraction, Raman spectra, dielectric constant vs temperature measurements and the photoluminescence of Eu³⁺ ions demonstrated that the prepared ceramics undergo a polymorphic phase transition (PPT, from orthorhombic to tetragonal phase transformation) with the rising LN content, and the PPT region locates at 0.05 ≤ x ≤ 0.06. The ferroelectric properties, Raman intensity ratios and photoluminescence intensity ratios show similar variations with the increasing LN content, all with a maximum value achieved at the PPT region. We believe that the close relationship among the ferroelectric properties, Raman intensity ratios, and photoluminescence intensity ratios is caused by the NbO₆ octahedral distortion. The photoluminescence of Eu³⁺ ion was discussed basing on the crystal‐symmetry principle and Judd‐Ofelt theory.     

The Relationship Between Phase Structure and Electrical Properties in (1 − x)(Bi0.5Na0.5TiO3–Ba0.5K0.5TiO3–BaTiO3)‐ xK0.5Na0.5NbO3 Quaternary Lead‐Free Piezoelectric Ceramics

(1 ? x)(0.85Bi_0.5Na_0.5TiO₃–0.11Ba_0.5K_0.5TiO₃–0.04BaTiO₃)‐ xK_0.5Na_0.5NbO₃ lead‐free piezoelectric ceramics with x = 0.00, 0.02, 0.03, 0.04, 0.05, and 0.10 were prepared by a conventional solid state method. A coexistence of rhombohedral (R) and tetragonal (T) phases was found in the system, which tended to evolve into pseudocubic symmetry when x increases. The x = 0.04 sample exhibited improved electrical properties: the dielectric constant ε_r = 1900 with the low loss tangents 0.06, the S_max/E_max of ~400 and ~460 pm/V under unipolar and bipolar electric field, respectively. Meanwhile, piezoelectric constant d₃₃ still maintained ~160 pC/N. These could be owed to the formation of polar nanoregions for relaxor phase.     

K0.5Na0.5NbO3‐Based Lead‐Free Transparent Electro‐Optic Ceramics Prepared by Pressureless Sintering

Transparent lead‐free electro‐optic (EO) ceramics (K_0.5Na_0.5) _0.9Li_0.1Nb_0.9Bi_0.1O₃ have been fabricated by pressureless sintering. The ceramics have a fine‐grained structure and cubic‐like symmetry. The comodification with Li and Bi induces a diffuse phase transition, causing the ceramics become relaxor‐like and contain polar nanoregions. Our results reveal that excess Bi₂O₃ can further enhance the changes in the crystal structure and dielectric behavior of the ceramics, causing them become more cubic‐like and more relaxor‐like, respectively. These can reduce the light scattering arisen from birefringence and domain walls, and thus improving the optical properties. For the ceramics added with 4 and 6 mol% excess Bi₂O₃, the optical transmittance reaches a high value of 60%–70% in the near‐infrared region. The ceramics also exhibits a good linear EO response, giving an effective EO coefficient of 30–40 pm/V.     

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.     

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.     

Large Strain Response in 0.99(Bi0.5Na0.4K0.1)TiO3–0.01(KxNa1−x)NbO3 Lead‐Free Ceramics Induced by the Change of K/Na Ratio in (KxNa1−x)NbO3

Influence of K/Na ratio in (K_xNa_1?x)NbO₃ on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi_0.5Na_0.4K_0.1)TiO₃–0.01(K_xNa_1?x)NbO₃ (BNKT–K_xNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point T_F?R to room temperature. Accordingly, giant strain of ~0.46% (corresponding to a large signal d₃₃* of ~575 pm/V) which is comparable to that of Pb‐based antiferroelectrics is obtained at a K/Na ratio of ~1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric–ergodic relaxor phase boundary in the BNKT–K_xNN system and the tolerance factor t of the end number (K_xNN). In situ high‐energy X‐ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field‐induced distortion from the pseudocubic structure.     

(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。     

Composition‐ and Temperature‐Dependent Large Strain in (1−x)(0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3)– xNaNbO3 Ceramics

Ternary solid solutions of (1 ? x)(0.8Bi_0.5Na_0.5TiO₃–0.2Bi_0.5K_0.5TiO₃)– xNaNbO₃ (BNKT–xNN) lead‐free piezoceramics were fabricated using a conventional solid‐state reaction method. Pure BNKT composition exhibited an electric‐field‐induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature. Accompanied with the ferroelectric‐to‐relaxor temperature T_F‐R shifted down below room temperature as the substitution of NN, a compositionally induced nonergodic‐to‐ergodic relaxor transition was presented, which featured the pinched‐shape polarization and sprout‐shape strain hysteresis loops. A strain value of ~0.445% (under a driving field of 55 kV/cm) with large normalized strain of ~810 pm/V was obtained for the composition of BNKT–0.04NN, and the large strain was attributed to the reversible electric‐field‐induced transition between ergodic relaxor and ferroelectric phase.     

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...     

Electrical Properties of a 0.95(Na0.5K0.5)NbO3–0.05CaTiO3 Thin Film Grown on a Pt/Ti/SiO2/Si Substrate

An amorphous phase was formed in a 0.95(Na_0.5K_0.5)NbO₃–0.05CaTiO₃ (NKN‐CT) film grown at 300°C, and a low‐temperature transient Ca₂Nb₂O₇ phase was formed in the film grown at 500°C. In films grown at high temperatures (≥600°C), secondary phases such as K_5.75Nb_10.85O₃₀ and K₄Ti₁₀Nb₂O₂₇ were developed without the formation of a NKN‐CT phase, probably because of Na₂O evaporation. The same secondary phases were formed in the film grown at 300°C and subsequently annealed at 850°C under an air atmosphere. However, a homogeneous NKN‐CT phase was formed in films grown at 300°C and subsequently annealed at 830°C–880°C under the K₂O and Na₂O atmospheres. Moreover, the film annealed at 830°C in particular exhibited good electric and piezoelectric properties, including a high dielectric constant of 747 with a low dissipation factor of 0.93% at 100 kHz, low leakage current density of 2.0 × 10^?7 A/cm² at 0.1 MV/cm, and high P_r and d₃₃ values of 15.4 μC/cm² and 124 pm/V at 100 kV/cm, respectively.     

Optical temperature sensing and luminescent switching properties in Pr/Er-doped (K0.5Na0.5)NbO3 materials

For optical temperature sensing materials, the emission and excitation bands are extremely critical to measure the temperature by fluorescence intensity ratio (FIR) technique. Singly Ln-doped optical temperature sensing materials exhibit very few emission bands, which greatly constraints their practical applications of FIR technique. Here, the fabricated Pr/Er co-doped (K_0.5Na_0.5)NbO₃ materials exhibited multi-color (red-green) and dual-mode (downshifting/upconversion) luminescence properties. The temperature sensitivity can be effectively tuned by choosing different emission or excitation bands. The optimized optical temperature sensitivity reached up to 0.0094 K⁻¹, much higher than that of most temperature sensing materials. Besides, the samples also showed excellent luminescence modulation properties based on the photochromic reaction. Under sunlight irradiation, the luminescent switching contrast (ΔR_t) of the samples reached more than 60%. These results may provide a guiding role in designing and modulating optical temperature sensing properties for multifunctional materials.