KNbO3 Single Crystal Research Articles

Analytical study of the ferroelectric properties of Fe-doped KNbO3 single crystal

We report herein the structural, dielectric, ferroelectric, and domain properties of an Fe-doped potassium niobate ([KNbO3] KNb1-xFexO3; x = 0.01 mol%) single crystal synthesized by a flux method. X-ray diffraction (XRD) results illustrate that the given sample adopts orthorhombic crystal symmetry with the space group Amm2. On submitting the sample to differential thermal analysis (DTA), two endothermic peaks were observed, the first at 224 °C and the second at 425 °C. Dielectric studies with respect to temperature from 30 to 500 °C showed that the Curie temperature (Tc) of the Fe-doped KNbO3 single crystal occurred at 435 °C. The measured dielectric loss of the crystal is in good agreement with other dielectric studies. The relationship between polarization and electric field (PE) shows that the spontaneous polarization (Ps) and coercive field (Vc) for the doped material are lower than those for an undoped KNbO3 single crystal, indicating a weakly ferroelectric nature of the sample. A trinocular microscope has been used to study the domain properties and revealed the formation of 60° and 90° domain walls. Furthermore, upon etching the single crystal with nitric acid (HNO3), etch pits and a twinning plane of the domains were formed. Finally, electric fields of 100, 500, and 1000 V/cm were applied to the Fe-doped KNbO3 single crystal surface, whereupon the nucleation and dissipation of new domains could be observed.

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.

Effect of etching on formation of 60° domain in KNbO3 single crystal

Domain study is a very useful tool to explain ferroelectric materials for its PotassiumNiobate (KNbO3) crystal were doped with Aluminum (Al). This Al-doped KNbO3 ferroelectric crystal further chemically etched using the etchant methyl alcohol. Trino cular m icro scopy m ethod was u sed to verify the ob servation s. In this study 60° domains domain walls and domain boundaries observed. Brief mechanism offorming 60° domain structure and domain wall discussed.

Nucleation and evaporation of new domains under the influence of successive electric field in Al-doped KNbO3 single crystal

Photographic proof of domain wall nucleation was originated on the surface of Al-doped KNbO3 single crystal. For the photographic proof, experiment were performed using a trinocular microscopy technique. Nucleation and evaporation of 60° and 90° domain were conducted under the applied electric field at room temperature which is consistent to the elastic theory of dislocation. An attempt was made by the dislocation model to clarify that the nucleation and evaporation of the domain walls was dependent on impurity dipoles. The appearance of nucleation and evaporation of domains under the influence of electric fields was interestingly studied in domain engineering.

Kinetics of ferroelectric domains investigated by etching technique in Al-doped KNbO3 single crystal

A single crystal sample of Al-doped Potassium Niobate (KNbO3) [KN] was produced using a high-temperature flux technique. The fully grown sample of Al-doped Potassium Niobate (Al: KNbO3) crystals were subjected to domain studies. The cleavage face of ferroelectric Al-doped potassium niobate (KN) crystals has been with chemically etching by methanol (CH3OH) and Nitric acid (HNO3). The domain structure of single crystal KN within the orthorhombic ferroelectric section at temperature is investigated by Trinocular microscopes. 60, 90°, and 180° ferroelectric domains show constant interaction and orientation on the grown surface of the crystal. The most aim of this study to investigate the orientation, formations, and motion of the domains after the etching.

Effect of methyl alcohol and nitric acid as an etchant on the motions of ferroelectric domains in Al-doped KNbO3 single crystal

ABSTRACT The domain structure of the single crystal KNbO3 (KN) in the orthorhombic ferroelectrics phase at room temperature is investigated using optical and trinocular microscope. The cleavage face of ferroelectric Al-doped KN crystals was chemically etched using CH3OH (Methanol). At room temperature and before etching, the 60°, 90° domains were observed on the crystal surface. A dislocation model is proposed to explain the nucleation and evaporation of the domain wall under the influence of the electric field, which can be used to study the changes in motion of ferroelectric domains. After etching with HNO3 (Nitric acid) and applying an electric field, the 180° domains were observed.

Synthesis, structural, dielectric and domain properties of Al-doped KNbO3 single crystal

Single-crystal samples of Al-doped potassium niobate (KNbO3) [KN] were synthesized by a high-temperature flux technique. X-ray diffraction analysis of the crystal suggests that a single phase with an orthorhombic structure was formed. The dielectric studies of the KN single crystal as a function of temperature (from 30 to 500 °C) show that the compounds underwent a phase transition, which was confirmed by thermal analysis of the DTA curve. The measured loss tangent of the crystal, which is in good agreement with the dielectric studies. The ferroelectric property of the crystal was confirmed by studying the hysteresis loop. In this study, several parameters, like the coercive field and spontaneous polarization, were measured and calculated. The domain study was carried out using a trinocular microscope, which reported the formation of the 60° domain line and dislocation etch pits. The effects of the electric field on the domain of the Al-doped KN single crystal were discussed.

Domain imaging in Fe-doped KNbO3 single crystal via trinocular microscopy and scanning electron microscopy

Abstract Trinocular microscopy with a high spatial resolution is a promising technique for studying the surface morphology of materials. The cleavage face of ferroelectric Fe-doped K niobate (KNbO3) crystals was chemically etched by using HNO3. Trinocular microscopy was used to study the surface morphology of the grown ferroelectric KNbO3 single crystals. The grown surface of the crystal with a 60° domain wall revealed the same interaction and orientations. The results indicate that the type of domain walls that occur in ferroelectric crystals depend on the symmetry of both the nonferroelectric and ferroelectric phases of the crystal.

Formation of a KNbO3 single crystal using solvothermally synthesized K2-mNb2O6-m/2 pyrochlore phase

A K2-mNb2O6-m/2 single crystal with a pyrochlore phase formed when the Nb2O5 + x mol% KOH specimens with 0.6 ≤ x ≤ 1.2 were solvothermally heated at 230 °C for 24 h. They have an octahedral shape with a size of 100 μm, and the composition of this single crystal is close to K1.3Nb2O5.65. The single-crystal KNbO3 formed when the single-crystal K2-mNb2O6-m/2 was annealed at a temperature between 600 °C and 800 °C with K2CO3 powders. When annealing was conducted at 600 °C (or with a small amount of K2CO3), the KNbO3 single crystal has a rhombohedral structure that is stable at low temperatures (< − 10 °C). The formation of the rhombohedral KNbO3 structure can be explained by the presence of the K+ vacancies in the specimen. The KNbO3 single crystal with an orthorhombic structure formed when the K2-mNb2O6-m/2 single crystal was annealed at 800 °C with 20 wt% of K2CO3.

Complete electroelastic set for the (YXt)-45° cut of a KNbO3 single crystal

A complete and consistent set (elastic, dielectric, and piezoelectric tensors) of a commercial lead-free (YXt)-45° cut KNbO3 single crystal is reported. These data were obtained using several samples and the resonance-antiresonance method. Particular attention was paid to the consistency of this delivered database. A genetic algorithm with an appropriate criterion was used. Electromechanical characterization revealed a high thickness coupling factor of approximately 60%. These properties make this single crystal a good candidate for several applications such as medical imaging. This complete set provides a basis for simulation designs of such devices integrating this piezoelectric lead-free material, especially for ultrasonic transducers.

Pressure and electric field effects on piezoelectric responses of KNbO3

The dielectric and piezoelectric properties of a KNbO3 single crystal under applied hydrostatic pressure and positive bias electric field are investigated using phenomenological Landau-Ginzburg-Devonshire thermodynamic theory. It is shown that the hydrostatic pressure effect on the dielectric and piezoelectric properties is similar to temperature, suggesting a common underlying mechanism for the piezoelectric anisotropy and its enhancement. The stable phase diagram of KNbO3 as a function of temperature and positive bias electric field is constructed. The maximum piezoelectric coefficient d33o* varying with temperature and electric field is calculated.

Piezoelectric anisotropy of a KNbO3 single crystal

Orientation dependence of the longitudinal piezoelectric coefficients (d33∗) of a KNbO3 single crystal has been investigated as a function of temperature by using the Landau–Ginzburg–Devonshire phenomenological theory. It is shown that the maximum of d33∗ is not always along the polarization direction of the ferroelectric phase. The enhancement of d33∗ along a nonpolar direction is attributed to a ferroelectric phase transition at which a polarization changes its direction. In the tetragonal phase, the maximum of d33t∗ at high temperatures is along the tetragonal polar direction and then changes its direction toward the polar direction of the orthorhombic phase when the temperature is close to the tetragonal-orthorhombic phase transition. The maximum of d33o∗ of the orthorhombic phase depends on both the high-temperature and low temperature ferroelectric phase transitions. In the rhombohedral phase, the maximum of d33r∗ is relatively insensitive to temperature due to the absence of any further phase transitions in the low temperature regime. These results can be generalized to the phase transitions induced by external electric field, pressure, and composition variations.

Thermodynamics and ferroelectric properties of KNbO3

The Landau–Ginzburg–Devonshire phenomenological theory is employed to model and predict the ferroelectric phase transitions and properties of single-domain potassium niobate (KNbO3). Based on the LGD theory and the experimental data of KNbO3 single crystal, an eighth-order polynomial of free energy function is proposed. The fitted coefficients are validated by comparing to a set of experimental measured values including phase transition temperatures, spontaneous polarization, dielectric constants, and lattice constants. The effects of hydrostatic pressure and external electric field on phase transition temperatures and piezoelectric coefficients are investigated. The free energy function may be used to predict ferroelectric domain structures and properties of KNbO3 bulk and films by phase-field approach.

Phase Matching Characteristics of Second-Harmonic Generation in Periodic 90°-Domain Structures of KNbO3

Phase matching characteristics of second-harmonic generation (SHG) in periodic 90°-domain structures of KNbO3 single crystal were extensively analyzed. Important parameters including types of interaction and the corresponding effective nonlinear optical coefficients, grating periods for quasi-phase matching (QPM), spectral bandwidths, and group velocity mismatches were estimated. The results show that KNbO3 with a specific internal structure of periodic 90°-domains can be used for efficient narrowband second-harmonic generation and group-velocity-matched broadband optical frequency doubling near the optical communication window.

Ferroelectric properties in KNbO3 thin films probed by optical second harmonic generation

Ferroelectric phase transitions and the structure of KNbO3 nanometer-thick films deposited by Metal-organic chemical-vapor deposition technique on a MgO(100) substrate are studied. Thin KNbO3 films of the average thickness from 20to150nm possess a grainlike structure with a lateral grain size of about 250nm. Such films reveal ferroelectric properties similar to KNbO3 single crystal. Temperature dependencies of optical second harmonic generation intensity show that the Curie temperature TC of the nanometer-thick KNbO3 films is about 30°C lower than the TC of KNbO3 single crystal, which is attributed to size effects in KNbO3 nanograins.

Growth and Characterization of KNbO3 Single Crystal by Vertical Bridgman Method

Single crystals of potassium niobate (KNbO3) were grown by the vertical Bridgman method. The crystal was cut into 3 × 3 × 0.7 mm3 plates, and dielectric constants and resonance characteristics were observed as a function of the magnitude of the electric field used in the poling treatment. With [001]pc poled crystal, which stands for [001] direction in pseudocubic crystal lattice, no large change in properties resulted even if the electric field was changed. However, with [101]pc poled crystal, a reduction in the dielectric constant, a shift to a higher resonance frequency, and an increase in the phase angle in the inductive reactance region were observed when the electric field became larger than approximately 70 V/mm. These results can be systematically explained by the relative directions of the dipole moment and the electric field, and the resulting movement.

Solid-Solution Structure and Piezoelectric Property of KNbO3 Ceramics Doped with Small Amounts of Elements

The origin of the improvement of the ferroelectric and piezoelectric properties of KNbO3 ceramics by the addition of small amounts of La and Fe has been investigated. The La/Fe-codoped KNbO3 ceramics formed a single (K1-xLax)(Nb1-xFex)O3 solid solution, and their unit-cell volume decreased significantly with an x of 0.002 mol, while they recovered at x=0.004. In this region, the lattice parameters of the a-b basal plane decreased and the elongation in the c direction toward apical oxygen in NbO6 octahedra occurred. These results were correlated with the enhanced polarization and piezoelectric properties measured for the x=0.002 specimen. The x=0.002 specimen demonstrated a kt of 0.48 which corresponds to approximately 70% of the kt value predicted for a KNbO3 single crystal as its maximized property. Furthermore, La/Fe-codoped KNbO3 ceramics with MnO2 addition were prepared to observe the effect of multivalent ion (Fe and Mn) doping into the KNbO3 structure on the properties. Synchrotron near-edge X-ray absorption spectroscopy (XANES) study revealed a mixture of k-edge spectra consisting of Mn4+/Mn3+/Mn2+, while Fe3+ maintained its valence state. The influence of valence fluctuation on the ferroelectric and piezoelectric properties of KNbO3 ceramics was discussed.

Research of Super-High Electromechanical Coupling Surface Acoustic Wave Substrates

The properties of surface acoustic wave (SAW) devices mainly depend on the choice of piezoelectric substrate materials. The important properties required for SAW substrates are a large electromechanical coupling coefficient (k2), small temperature coefficient of delay (TCD), and low propagation loss, etc. Especially, super-high coupling substrates are very important for obtaining wide-band SAW filters, wide-band voltage controll oscilater (VCO) and delay lines with small insertion loss. KNbO3 single crystal has an extremely large electromechanical coupling coefficient (k2=53%) and zero TCD at approximately room temperature. Large-scale single-crystal growth technologies are now under investigation. In this study, the propagation characteristics of SAW in PZNT and KTP substrates are investigated experimentally in order to obtain higher coupling substrates than those of KNbO3. The experimental results show that the substrates have large k2. Also, theoretical SAW propagation characteristics with a k2 of about 100% are analyzed and their physical phenomena are discussed.

Low temperature X-ray luminescence of KNbO3 crystals

We have studied X-ray luminescence of KNbO3 single crystal. The 575 nm luminescence band has been studied in the temperature range of 15–45 K. The quenching parameters were found to be Q=12±3 meV and ν=4×1011 s−1. No luminescence has been observed under heavy ion excitation (86Kr ions, 8.63 MeV/amu) even at 15 K.

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.