Ruiping Wang

High-pressure synthesis and ferroelectric properties in perovskite-type BiScO3–PbTiO3 solid solution

Stabilization of a perovskite-type solid solution (1−x)u200aPbTiO3–xu200aBiScO3 with x⩾0.45 was demonstrated by high-pressure synthesis, and the phase diagram and the ferroelectric properties of the solid solution were investigated. The crystal symmetry of the perovskite subcell change in turn from tetragonal, to rhombohedral, to pseudocubic, to monoclinic, and then to triclinic as x increases. It was found that the tetragonal, rhombohedral, and pseudocubic phases are ferroelectric, while the monoclinic phase is not. In the morphotropic phase boundary in the vicinity of x=0.37 between tetragonal and rhombohedral phases, the maximum electromechanical coupling factor and the minimum coercive electric field were just like those observed in other Pb-based ferroelectric perovskites. In addition, relaxor behavior in the dielectric constant was observed in the vicinity of x=0.5.

Universality in phase diagram of (K,Na)NbO3–MTiO3 solid solutions

Detailed phase diagrams are necessary for improving the temperature stability of piezoelectric properties of (Na,K)NbO3-based solid solutions. We report the phase diagrams of the (1−x)Na0.5K0.5NbO3–xMTiO3 (M=Pb, Ba, Sr, Ca, and Bi0.5Li0.5) solid solutions over a broad temperature range of 10u2002K≤T≤∼770u2002K. It is found that the phase diagrams show several universal characteristics: (a) Curie temperature TC-T decreases sharply at low x range; (b) the relation between TC-T and x deviates from Vegard’s law; and (c) the tetragonal-orthorhombic phase transition temperature TT-O and the orthorhombic-rhombohedral phase transition temperature TO-R are almost independent of M. We suggest that valence mismatch is the main reason for the rapid decrease in TC-T and the deviation of TC-T from Vegard’s law. It is also found that TC-T is mainly affected by the A-site cations, whereas TT-O and TO-R are mainly affected by the B-site cations.

Dielectric and Piezoelectric Properties of Barium-substituted Sr1.9Ca0.1NaNb5O15 Ceramics

Highly dense piezoelectric ceramics of tungsten bronze-type (Sr1.9Ca0.1)1-0.5xBaxNaNb5O15 (where x=0.1–0.8) were prepared by spark plasma sintering. The crystallographic parameters, dielectric behaviors and piezoelectric properties of the sintered ceramics were investigated, and the effects of the Ba substitution on these electrical properties were discussed. The structural analysis and the electrical property measurements indicate a morphotropic phase boundary (MPB)-like phenomenon at x=0.4–0.5. In all compositions, a diffuse phase transition and a relaxor behavior are observed. The electrical properties are found to be crystallographically dependent.

MPB Phase Diagram and Ferroelectric Properties in the PbTiO 3 -BiScO 3 System

Perovskite solid solutions (1 m x )PbTiO 3 (PT)- x BiScO 3 (BS) were prepared under ordinary pressure. Single phase tetragonal perovskite was obtained up to the composition x = 0.375, and morphotropic phase boundary (MPB) from tetragonal to rhombohedral phase was also situated on this composition. The dielectric measurements suggested existence of critical compositions around both x = 0.15 and x = 0.3. The Curie temperature ( T c ) was higher than that of PT at x = 0.1-0.2. Below and above x = 0.3, the dielectric constant at T c increased and decreased with increasing BS content, respectively. These dielectric behaviors seemed to be associated with the connectivity of TiO 6 octahedron and ScO 6 octahedron in the perovskite structure.

Broadband inelastic light scattering of a relaxor ferroelectric 0.71Pb(Ni1/3Nb2/3)O3-0.29PbTiO3

Brillouin and Raman scatterings of a 0.71Pb(Ni1∕3Nb2∕3)O3-0.29PbTiO3 single crystal have been measured to investigate broadband inelastic spectra. The two different central peaks related to fast and slow relaxation processes have been observed separately. These two processes are attributed to the thermally activated switching of polarization in polar nanoregions. By the analysis of modified superparaelectric model, the activation energies of fast and slow relaxation processes are determined to be 3.66×103 and 4.31×102K, respectively. The fast process with the lower activation energy probably originated from 180° switching, whereas the slow one with the higher energy from non-180° switching.

Effect of ZnO buffer layer on the quality of GaN films deposited by pulsed laser ablation

Abstract ZnO, GaN and ZnO/GaN films have been deposited on c-cut sapphire wafers by Nd:YAG pulsed laser deposition to develop preparation techniques of multi-layer devices in optoelectronics. The quality of the films was investigated as functions of deposition temperature and pressure by means of X-ray diffraction, reflection high energy electron diffraction, scanning electron microscopy and optical transmission spectroscopy. It was found that GaN films could be epitaxially grown on ZnO buffered sapphire substrates even at a low substrate temperature of 300 °C and inactivated N2 flow, confirming the significant role of ZnO as a buffer layer. The crystallinity, surface morphology and optical transmittance of these films could be improved with increasing deposition temperature and were optimized at 800 °C. The optical band gaps of ZnO and GaN were estimated from the optical transmission spectra to be 3.31 and 3.40 eV, respectively.

Dynamical Properties of Polar Nanoregions of Relaxor Ferroelectric Pb(Ni1=3Nb2=3)O3-0.29PbTiO3

Dynamical properties of the polar nanoregions (PNRs) of a relaxor ferroelectric Pb(Ni 1/3 Nb 2/3 )O 3 –0.29PbTiO 3 single crystal are studied by using Brillouin scattering to elucidate the mechanism of the diffuse phase transition. The temperature dependence of Brillouin scattering shows anomalies at three temperatures: at T d related to the appearance of the PNRs, at T * associated with the growth of the PNRs, and at T f related to the macroscopic structural phase transition; all these temperatures are closely related to the dynamical behavior of the PNRs. The thermal hysteresis around T f in the longitudinal acoustic properties can be interpreted by considering the growth process of the PNRs. From the anomalous acoustic properties, the relaxation time of the polarization fluctuation is evaluated. It is found that the relaxational process responsible for the central peak originates from the polarization fluctuation that is coupled with the longitudinal acoustic mode.

Phase Diagram of the (Na0.5K0.5)NbO3-ATiO3 Solid Solution

To elucidate mechanisms responsible for the enhanced piezoelectric properties in the potassium-sodium niobate-based solid solutions, phase diagram of the (1 − x) (Na0.5K0.5)NbO3-xATiO3 (A = Sr, Ba, and Pb) systems has been established according to the dielectric constant measurement and the crystal structure analysis results. A tetragonal/orthorhombic morphotropic phase boundary (MPB) is found around x∼ 0.05, where the piezoelectric properties were enhanced. Rietveld refinements show that the tetragonal/orthorhombic MPB is the same as that found in the lead zinc niobate-lead titanate system.

Preparation of parallel capacitor of epitaxial SrTiO3 film with a single-crystal-like behavior

Epitaxially grown SrTiO3 thin films with single-crystal-like dielectric behavior were obtained by the chemical mechanical planarization method. A parallel capacitor of a SrTiO3 film with excellent insulation was fabricated between YBa2Cu3O7−δ electrodes by pulsed laser deposition. Dielectric constants of the parallel capacitor with a thickness of 300–600 nm are 330 and >20u200a000 at room temperature and 4.2 K, respectively, and obey the Curie–Weiss law above 150 K, being identical to the behavior of SrTiO3 single crystals. The microshort-free insulating film may aid in the development of oxide superconductor multilayer devices, such as superconducting quantum interference devices, three-terminal transistors, and field effect transistors.

Formation of Morphotropic Phase Boundary in (Na0.5K0.5)NbO3–BaZrO3–(Bi0.5Li0.5)TiO3 Lead-Free Piezoelectric Ceramics

For the development of lead-free piezoelectric ceramics, which have a tetragonal–rhombohedral morphotropic phase boundary (MPB), 0.90(Na0.5K0.5)NbO3–xBaZrO3–(0.10 - x)(Bi0.5Li0.5)TiO3 ceramics have been synthesized by the conventional solid state reaction method. A phase diagram was summarized from the X-ray diffraction pattern and the dielectric measurements. It is found that 0.90(Na0.5K0.5)NbO3–xBaZrO3–(0.10 - x)(Bi0.5Li0.5)TiO3 ceramics have an MPB between x = 0.07 and 0.08. In the vicinity of the MPB, it was observed that the dielectric and piezoelectric properties were enhanced.