Waveguides in LiNbO3 by varying angle implantation of HE

A severe problem in He⁺ implanted LiNbO₃ waveguides is the tunneling of electromagnetic energy through the ion induced potential barrier of lowered refractive index. We drastically reduced this leakage, especially occuring for higher order modes, by varying the impact angle during the implantation thus producing a broader barrier.     

Dislocation toughening in single-crystal KNbO3

The growing research interest in dislocation-tuned functionality in ceramics is evident, with the most recent proofs-of-concept for enhanced ferroelectric properties, electrical conductivity, and superconductivity via dislocations. In this work, we focus on dislocation-tuned mechanical properties and demonstrate that, by engineering high dislocation densities (up to 10¹⁴ m⁻²) into KNbO₃ at room temperature, the fracture toughness can be improved by a factor of 2.8. The microstructures, including dislocations and domain walls, are examined by optical microscopy, electron channeling contrast imaging, piezo-response force microscopy, and transmission electron microscopy methods to shed light on the toughening mechanisms. In addition, high-temperature (above the Curie temperature of KNbO₃) indentation tests were performed to exclude the influence of ferroelastic toughening, such that the origin of the toughening effect is pinpointed to be dislocations.     

凝胶燃烧法制备KNbO3纳米粉体

以自制的水合氧化铌(Nb2O.5nH2O)为铌源,硝酸钾(KNO3)为钾源,采用柠檬酸凝胶-燃烧法制备出铌酸钾(KNbO3,KN)粉体。通过TG/DSC研究了前躯体粉末的热分解过程;借助XRD和TEM对样品的晶体结构、微观形貌和粒径进行了表征。结果表明,过多的柠檬酸和羧酸盐在330℃附近分解成碳酸盐,在600℃时KN相开始形成,没有中间相产生。煅烧温度大于600℃即可获得分散良好、粒度均匀、形状规则的纳米KN粉体。随着煅烧温度的升高,KN粉体的结晶度提高,颗粒增大。     

Defect-induced superior piezoelectric response in perovskite KNbO3

In this work, for CuO-modified perovskite KNbO₃ (KN-xCuO), by selecting the appropriate composition and external field conditions (including direct/alternating E-field, time, temperature and bipolar cycles), we report the superior piezoelectric responses (incipient large-field piezoelectric responses d₃₃* = 1994 pm/V, S_max>0.3 %). This work indicates that the unusual ferroelectric and electro-strain behaviors may be attributed to the discrepancies of characteristic frequency for defect dipoles transformation (Cu_Nb-V_ab1 and Cu_Nb-V_ab2) and ferroelectric domains switching. Different external fields affect the transformation process of defect dipoles, leading to the different constructing and restoring behaviors during the E-field loading and unloading process. After selecting the appropriate external field conditions, the large and stable electro-strain behaviors (maximum electro-strain S_max>0.3 % exhibits 2.6 % decline after 10⁵ cycles at room temperature) are observed for the d.c.-poled and one-year-aged x = 0.015 samples. This research may provide a paradigm for designing the high-performance KNbO₃-based perovskite ferroelectrics by exploring the greatest potential of dynamic defect dipoles.     

Formation of cBN nanocrystals by He+ implantation into hBN

The structural modifications of polycrystalline hexagonal boron nitride implanted with He⁺ beams at energies between 200 keV and 1.2 MeV to fluences of 1.0 × 10¹⁷ ions cm^? 2 were investigated using micro-Raman spectroscopy. The measured Raman spectra show evidence of implantation-induced structural transformations from the hexagonal phase to nanocrystalline cubic boron nitride, rhombohedral boron nitride and amorphous boron nitride phases. The first-order Longitudinal-Optical cubic boron nitride phonon was observed to be downshifted and asymmetrically broadened and this was explained using the spatial correlation model coupled with the high ion implantation-induced defect density.     

Detailed index profiles of ion implanted waveguides in KNbO3

Planar optical waveguides have been produced in KNbO₃, by He ion implantation at 2.0 MeV. The depth profiles for the two indices (n_a and n_c) have been determined using a dark mode reflectivity calculation. They show a pronounced anisotropic behaviour. For a dose of 2 × 10¹⁶ cm^-2 the nuclear damage peak has an index decrease ∼10% for n_a, and ∼7% for n_c, showing a sensitivity to nuclear damage at least three times that of LiNbO₃. The electronic damage region has ∼0.5% decrease for n_a, but an increase for n_c which is larger at shorter wavelengths. This anisotropic and dispersive behaviour is comparable with other implanted niobates, and a similar damage mechanism of radiation-enhanced diffusion is suggested.     

Infrared spectroscopy of single-wiain KNbO3 in the tetragonal phase

Owing to the development of an original technique of crystal growth and thermal gradient poling, large single-domain samples of KNbO₃ have been prepared at high temperature in the tetragonal phase. Infrared reflection spectra are reported for the first time in this phase and compared with similar data previously obtained in isomorphous BaTiO₃.     

KNbO_3粉体的低温合成研究

以尿素为燃烧剂,采用低温燃烧法在较低的温度下合成了纯相的KNbO3粉体。采用XRD、SEM和TG-DSC研究了KN晶相的形成过程。结果表明,采用该法可在550℃下合成纯相的KN粉体,当煅烧温度升高至700℃时,立方相的KN粉体转变成了正交相的KN粉体。本研究还就工艺参数对粉体物相的影响进行了讨论。     

Preparation and study of KNbO3 PTC ceramics

In KNbO₃ ceramics rendered semiconductive by additions of K₂O-GeO₂ glasses two PTC anomalies are observed: at the Curie point and in the vicinity of the orthorhombic-tetragonal phase transition. 'These results are explained on the basis of Heywang-Jonker model of the PTC effect modified to take account of intergranular layers of glassy phase and spontaneous polarization in grains.     

The conductance analysis of ferroelectric KNbO3 single crystal

The complex impedance of KNbO₃ single crystal has been investigated at various frequencies from 10 Hz to 10 MHz and at various temperatures from 30°C to 550 °C. The frequency dependence of the impedance shows the characteristic features of the universal dielectric response and is described by an empirical Jonscher type relaxation function. High-frequency conductance dispersion was observed near the Curie temperature (T_c = 430 °C) at the frequency of 1.5 MHz. An equivalent circuit model based two parallel G-C circuits was adopted to describe the conductance relaxation behavior observed below and above T_c = 430 °C. The conductance relaxation was found as two types of dispersions. One may be caused by intrinsic polarization, the other by mobile carrier. This is attributed to a circuit element that includes the capacitance associated with the ferroelectric polarization processes.     

Theory of ferroelectricity and refractive indices in ferroelectric phases of KNbO3

The physical origin of the ferroelectricity and refractive indices in tetragonal, orthorhombic and rhombohedral phases of KNbO₃ are discussed quantitatively from the dipole-dipole interaction caused by the strong local field acting on the constituent ions. In this microscopic model, all the ions, except the barium, are assumed to have the spontaneous shifts. The results of calculations point out the important role of the lattice deformations and the electronic polarizabilities of the constituent ions. It is found that the ferroelectricity and the refractive indices are in good agreement with the experimental data.     

Morphology and crystallinity of KNbO3-based nano powder fabricated by sol–gel process

Lead-free KNbO₃ (KN) powder was fabricated by sol–gel process from metal alkoxides. KN precursor solutions were prepared by different preparation conditions such as the order of reflux process for alkoxides and the kinds of solvent. KN powders were sintered at 900 °C with a heating rate of 10 °C/min. Single phase KN powder was obtained using precursor solution prepared by reflux at 120 °C. We considered that the crystallinity of KN powder was affected by the dimer in Nb-pentaethoxide of the starting chemicals. On the other hand, grain shape of the KN powder also depended on the existence of secondary phase K₄Nb₆O₁₇, and the grain size of the fine powder fabricated from precursor solution prepared by reflux at 80 °C using 2-methoxethanol and ethanol solvents was estimated to be about 500 nm and 1 μm, respectively.     

Preparation of KNbO3 powders by sol-gel method using water-soluble potassium and niobium compounds as precursors

In the paper the full sol-gel process of KNbO₃ powder preparation, using KOH, Nb₂O₅ and K₂CO₃ as initial substrates as well as oxalic and citric acid as a chelating agent is presented. It was also confirmed that a mixture of compounds obtained during calcination of KOH and Nb₂O₅, mixed in adequate proportions, could be source of Nb ions in the sol-gel process. The crystal and electronic structure parameters of the manufactured powder proper for KNbO₃ compound were established using XRD, XPS and UV-VIS techniques.     

Simulation of characteristics of KNbO3/diamond surface acoustic wave

High-frequency Surface Acoustic Wave (SAW) Devices based on diamond that has been realized to date utilize the SiO₂/ZnO/diamond structure, which shows excellent characteristics of high phase velocity over 10,000 m/s with zero temperature coefficient, and it has been successfully applied to high-frequency narrowband filters and resonators. In this paper, the characteristics of KNbO₃/diamond with the assumption that the KNbO₃ film is single crystal have been studied by theoretical calculations to find higher coupling coefficient condition. Calculations are made for the phase velocity and electromechanical coupling coefficient of the Rayleigh wave and its higher mode Sezawa waves. As a result, KNbO₃/Ground/diamond is found to offer extremely high electromechanical coupling coefficient up to 38% in conjunction with high phase velocity of 12,600 m/s. This characteristic is suitable for ultra-wide bandwidth applications in high-frequency SAW devices.     

Influence of sintering conditions on piezoelectric properties of KNbO3 ceramics

A homogeneous KNbO₃ (KN) phase was formed in specimens that were sintered at 1020 °C and 1040 °C, without formation of the K₂O-deficient secondary phase, indicating that the amount of evaporation of K₂O during sintering was very small. However, the KN liquid phase was formed during sintering and assisted the densification of the KN ceramics. A dense microstructure was developed in the specimen sintered at 1020 °C for 6 h and abnormal grain growth occurred in this specimen. A similar microstructure was observed in the specimens sintered at 1040 °C for 1.0 h. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The KN ceramics sintered at 1020 °C for 6 h, which showed a large relative density that was 95% of the theoretical density, exhibited promising electrical properties: ɛ^T₃₃/ɛ_o of 540, d₃₃ of 109 pC/N, k_p of 0.29, and Q_m of 197.     

A critical field study of ferroelectric domain in Al-doped KNbO3 single crystal

-Ferroelectric domain engineering is important for studying the dipolar structure of these materials. This dipolar structure is formed because of defects and imperfections produced using various doping. The KNbO₃ ferroelectric single crystal was prepared using flux methods. Al₂O₃ aluminum trioxide (Al) was used as a dopant in a KNbO₃ crystal. The domain structure of the crystal was studied by applying electric fields of 50 V/cm, 60 V/cm, 70 V/cm, 80V/cm, and 100 V/cm in an Al-doped KNbO₃ single crystal. Furthermore, the critical field on which domains were nucleated after applying the electric field, was also studied. This work confirmed that the critical field of nucleation of the new domain in an Al-doped KNbO₃ single crystal was 70 V/cm, which is slightly less than that of pure KNbO₃ single crystal.     

Colorimetry technique on BaTiO3 and KNbO3 ceramics to determine the phase transition

In this work, colorimetry was used in order to identify the phase transition of BaTiO₃ and KNbO₃ ceramic. The transition of the structure produces a change of color of the ceramic bulks because their optical properties change. The transition phase was detected with a color sensor and reflectance method on ceramic bulks. CIE L*a*b* and CIE L*u*v* color spaces were used to measure the color changes reported. Experimental results showed that the changes of color are suitable for detecting the phase transition in both perovskite structures. The phase transition in the proposed ferroelectric materials was also measured by means of the dielectric constant method and the results obtained were in agreement with the temperatures reported for phase transition changes on BaTiO₃ and KNbO₃. Therefore, the colorimetry technique proposed in this paper proved to be suitable for detecting structure changes as a relationship between color properties and temperature.     

Chemical stability of KNbO3, NaNbO3, and K0.5Na0.5NbO3 in aqueous medium

In recent years, potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) has become popular and promising among perovskite lead‐free piezoceramic systems. In this study, the chemical stability of KNN powders in aqueous medium was investigated as a function of pH, time, and powder surface area. To better understand the dissolution behavior of the complex KNN stoichiometry, subconstituents such as potassium niobate (KNbO₃, KN) and sodium niobate (NaNbO₃, NN) were investigated separately first. Results showed that all of the cations in the structure underwent dissolution in different values. Indicating that KNN undergoes incongruent dissolution in aqueous medium, the dissolution of A site cations was higher at lower pH while the dissolution of B site cations increased at higher initial pH. The order of released cation concentrations (C_A1 = K > C_{A2 = Na} > C_B = Nb) fits with inverse relationship of cation field strength (FS) order, B = Nb⁵⁺_FS > A₂ = Na⁺_FS>A₁ = K⁺_FS, at pH 4, 7 and 10 for NN, KN, and KNN. Calculated diffuse layer thickness from the ICP data confirmed to outer amorphous layer in TEM image. Also, the ratio of normalized cation concentration versus surface area of powders showed that incongruent dissolution kinetic was driven by the diffusion step.     

Band-edge phenomena and spontaneous polarisation above and below Tc FOR BaTiO3 and KNbO3

The band-edge phenomena and the polarisation induced effects are analysed through the Bloch orbital interactions above and below T_c.