Haiwu Zhang

Orientation dependence of electrical properties of large-sized sodium potassium niobate lead-free single crystals

Lead-free (K0.25Na0.75)NbO3 (KNN25/75) single crystals with dimensions of O30 × 10 mm were successfully grown by a top seeded solution growth technique (TSSG). The X-ray diffraction pattern shows that the as-grown crystals possess an orthorhombic perovskite structure. The concentrations of K, Na, and Nb elements in the as-grown crystal were measured by X-ray fluorescence analysis. The dielectric, ferroelectric and piezoelectric properties of KNN25/75 crystals with different orientations, i.e., pseudocubic (100) and (110) ((100)pc and (110)pc), were investigated. A higher piezoelectric constant d33 ~ 145 pC N−1 and electromechanical coupling coefficient kt ~ 69% were observed along the (100)pc orientation compared with d33 ~ 70 pC N−1 and kt ~ 51% along (110)pc. However, when a bipolar electric field was applied, a two times higher strain value can be achieved in crystals oriented along (110)pc than along (100)pc. These orientation dependent physical properties can be explained in the framework of domain engineering and the switching effect.

Growth and orientation dependence of electrical properties of 0.92Na0.5Bi0.5TiO3-0.08 K0.5Bi0.5TiO3 lead-free piezoelectric single crystal

A 0.92Na0.5Bi0.5TiO3-0.08 K0.5Bi0.5TiO3 (0.92NBT-0.08KBT) lead-free piezoelectric single crystal with dimensions of Φ 35 × 10 mm was successfully grown by the top-seeded solution growth method. The effective segregation coefficient for K was calculated to be 0.27. An x-ray powder diffraction measurement showed that the as-grown crystal possesses a rhombohedral perovskite structure at room temperature. The room-temperature dielectric constants for poled 〈001〉, 〈110〉, and 〈111〉 oriented crystal samples are found to be 683, 567, and 435 at 1 kHz, respectively. The (Tm, ɛm) values for 〈001〉, 〈110〉, and 〈111〉 oriented crystals are (316 °C, 4318), (317 °C, 4160), and (318 °C, 4348) at 1 kHz, which indicate that the dielectric parameters of the as-grown crystals show weaker anisotropy. The curves, ɛ(T), for the three crystallographic orientations show two anomalies at about 170 and 320 °C, respectively, relating to the ferroelectric-antiferroelectric phase and the antiferroelectric-paraelectric phase. There is a...

Enhanced ferroelectric properties and thermal stability of nonstoichiometric 0.92(Na0.5Bi0.5)TiO3-0.08(K0.5Bi0.5)TiO3 single crystals

Bi deficient, Mn doped 0.92(Na0.5Bi0.5)TiO3-0.08(K0.5Bi0.5)TiO3 single crystals were grown by carefully controlled top-seeded solution growth method. Local structures were investigated by transmission electron microscopy. The site occupation and valence state of manganese were characterized by electron paramagnetic resonance spectrum. The leakage current density in the as-grown single crystals is effectively depressed. The introduced defect complexes suppress the temperature induced phase transformation, increasing the depolarization temperature (165 °C) and thermal stability of ferroelectric properties.

Quantifying the Microstructures of Pure Cu Subjected to Dynamic Plastic Deformation at Cryogenic Temperature

A pure Cu (99.995 wt%) has been subjected to dynamic plastic deformation at cryogenic temperature to a strain of 2.1. Three types of microstructures that are related to dislocation slip, twinning and shear banding have been quantitatively characterized by transmission electron microscopy (TEM) assisted by convergent beam electron diffraction (CBED) analysis. Microstructures originated from dislocation slip inside or outside the shear bands are characterized by low angle boundaries ( ) up to the maximum angle of 9 degrees. The quantitative structural characteristics are compared with those in conventionally deformed Cu at low strain rates, and allowed a quantitative analysis of the flow stress-structural parameter relationship.

Electric field and temperature-induced phase transition in Mn-doped Na1/2Bi1/2TiO3-5.0 at.%BaTiO3 single crystals investigated by micro-Raman scattering

A micro-Raman scattering technique was used to investigate the electric-field and temperature dependent phase stability of Mn-doped Na1/2Bi1/2TiO3-5.0at. %BaTiO3 single crystal. The Ti–O mode was found to exhibit a slight shift at a low electric field (E = 10 kV/cm) and splitting at higher electric field (E ≥ 30 kV/cm), ascribed to field-induced local distortion and phase transition, respectively. The temperature-dependent Raman scattering was also measured over a wide range of 150–800 K to study the phase stability of poled samples. A new Raman mode at about 200 cm−1 and an anomaly in intensity of the Ti-O modes were detected at 390 K, indicating a ferroelectric to antiferroelectric phase transition. The frequency shift of TiO6 octahedral modes implied a transition to a paraelectric state at 550 K. Furthermore, the Ti-O and TiO6 octahedral modes were found to be sustained in the high-temperature paraelectric state.

Giant strain and electric-field-induced phase transition in lead-free (Na0.5Bi0.5)TiO3-BaTiO3-(K0.5Na0.5)NbO3 single crystal

A lead-free single crystal 0.92(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.02(K0.5Na0.5)NbO3 (NBT-6BT-2KNN) with dimension of Φ35 mm × 10 mm is grown by a precisely controlled top seeded solution growth method. The 〈001〉 oriented single crystals have excellent piezoelectric properties with a giant strain of 0.83% at 28 kV/cm. Application of an electric-field ≥14 kV/cm leads to a phase transition from pseudocubic to coexistence of tetragonal and pseudocubic. A strong ferroelectric domain texture occurs during the phase transition. Furthermore, the variation of tetragonal phase fraction agrees well with the macroscopic strain curve, demonstrating that the induced tetragonal phase plays a critical role in the high strain property.

Direct observation of monoclinic ferroelectric phase and domain switching process in (K0.25Na0.75)NbO3 single crystals

In situ observations of the phase symmetry and domain evolution of (K0.25Na0.75)NbO3 single crystals under bipolar electric fields have been performed for both pseudocubic (110) and (001) orientations using polarized light microscopy and X-ray diffraction. The macroscopic symmetry was identified to be a monoclinic C type phase with the spontaneous polarization direction close to . The electric field-driven domain morphology and evolution model were also presented. It was found that, under an antiparallel electric field, the approximately 180° domain switching process in a pseudocubic (101) sample consists of three steps: two ~60° domain switching and a ~90° domain switching, and the ~90° domain switching process in the pseudocubic (001) sample is composed of two ~60° switching steps.

Pressure-enhanced thermal stability against eutectic crystallization in Al-based metallic glasses

The pressure effect on thermal stability against eutectic crystallization in an Al84Ni10Ce6 and an Al82.5Ni5Y8Co2Zr2.5 metallic glasses was investigated in a pressure range up to 1.3 GPa. It was found that the applied pressure enhances the eutectic crystallization temperature for both glasses

Ultrahigh ferroelectric response in Fe modified 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3 single crystals

Single crystals of x at% Fe + 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3 (x%-Fe:NBBT5, x = 0.1, 0.2 and 0.5) with ultrahigh ferroelectric response were developed by introducing defect associations. The very large field-induced bipolar and unipolar strains, i.e., Smax ∼ 1.1%, emax/Emax ∼ 1300 pm V−1, d33 ∼ 600 pC N−1 and permittivity tunability ∼120%, demonstrate that these crystals are promising candidates as lead-free ferroelectrics. The presence of ferromagnetic properties further provides their new application potential as multiferroic materials. The defect chemistry and domain structure were studied systematically. The effects of microscopic defect functional centers on macroscopic properties were discussed in detail.

Optical dispersion and interband transition in Na0.5Bi0.5TiO3-x%BaTiO3 lead-free relaxor ferroelectric single crystals

Optical properties of A-site disordered Na0.5Bi0.5TiO3-x%BaTiO3 (NBT-xBT) single crystals with three typical compositions were studied systematically. Refractive index as a function of wavelength was measured by Spectroscopic ellipsometry, and modified Sellmeier dispersion equations were obtained by the least square fitting. High optical transmittance (65%) over a wide transparent region (from 400 to 6000 nm) has been found in NBT-7.5BT, much higher than that of NBT and NBT-5BT. Optical band gap energies were calculated using the absorption coefficient through Tauc equation. With increasing BT content, refractive index and absorption coefficient decrease, while transmission and band gap energy increase.