Papers by Author: Wen Wang

Abstract: The electrochemical deposition of thiophene and CdSe on TiO₂/ITO by cyclic voltammetry(CV) technique was carried out to study the influence of conditions on the preparation and photophysical performance of poly(thiophene) (PTh)/CdSe double layers electrode. The chemical structure of the obtained polymer was characterized by FT-IR, Raman, X-ray diffraction, UV-vis spectroscopy. The morphology was observed by electronic scanning microscope (SEM) which showed nano-grain shape. Finally the photovoltaic performance of the solar cell which was composed of PTh/CdSe/TiO2 composite film used as working electrode, Pt used as counter electrode was investigated. And mechanism of photogenerated electron of PTh/CdSe electrode was proposed. The results showed that TiO₂ electrode modified by PTh/CdSe composite film had good response on irradiation, which indicated the PTh/CdSe was promising dye in solar cell.

Abstract: The present investigation reports on the preparation of BiFeO3 nanopowders by a sol-gel method using acetylacetone as a stabilizer. Single-phase BiFeO3 nanopowders without any impurity or amorphous phases were obtained when the precursor was thermal treated at temperatures as low as 400 oC for 2 h. Acetylacetone (acac) plays an important role on lowering the formation temperature of pure phase BiFeO3 nanopowders. It is found that Bi/acac molar ratio of 1/30 was favorable for a stable sol and for the lowest crystallization temperature of pure BiFeO3 nanopowders. X-ray diffraction and Fourier transform infrared spectroscopy revealed that thermally induced crystallization process of BiFeO3 nanopowders from Bi-Fe polymeric precursor. When the thermal treated temperature was below 200 oC, only amorphous phase existed. With the temperature increasing up to 300 oC, crystallized phase, carbonate, were detected. After annealed at 400 oC, Bi-Fe precursor totally changed to rhombohedral BiFeO3 nanopowders. Scanning electron microscopy characterized morphologies of BiFeO3 nanopowders calcined at 400 oC and 500 oC. The ferroelectric transition of BiFeO3 nanopowders at 827 oC has been detected by differential thermal analysis.

Abstract: Bi4-xLaxTi3O12 (BLT, x=0, 0.75, 1, 2) ceramic powders were prepared with sol-gel and solid phase reaction processes, respectively. BLT powders were synthesized after decarbonization at 300°C for 1h followed by calcination at 800°C for 2h. The ceramics synthesized with cold isostatic pressing and sintering at 1150°C exhibit a perovskite-like structure. The influence of sintering parameters and La doping content on density, microstructure and dielectric properties of the BLT were discussed. With the increase of La doping content in Bi4-xLaxTi3O12, dielectric constant increases and dielectric loss gradually decreases. The dielectric constant and dielectric loss of Bi4-xLaxTi3O12 (x=2) are 132.7 and 0.012 at 100 KHz, respectively.

Abstract: The microstructure and ferroelectric domains of SrBi2Ta1.6Nb0.4O9 ceramics were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). The X-ray diffraction patterns show that the lattice constants a and b decrease, and c increases by doping with Nb into SBT sample. Accordingly, it has large strain and lattice distortion in the lattice This suggests that the Nb atoms partially occupy the location of the Ta atoms in the lattice. From TEM observations, the grains show (008) preferred orientations in the sample, which agrees well with the XRD results. The 90° domain walls are identified by the 90° rotation relationship of the electron diffraction pattern about the [001] zone axis. The 180° domain walls and anti-phase boundaries (APBs) in Nb-doped SBT ceramics are also observed, which are irregularly shaped and highly curved. The traditional α-fringes can be found in the Nb-doped SBT ceramics, which are the evidence of large strains in the lattice.

Abstract: In this research, SrBi2+xTa2O9 (SBT) powders with different Bi content were prepared using conventional solid-state reaction method. The SrBi2+xTa2O9 ceramics (x=0 ,0.2 ,0.4 ,0.6) were fabricated by the cold isostatic compaction and sintering in the air. Microstructure and dielectric, ferroelectric properties of SBT ceramics were investigated by XRD, SEM, EDAX and dielectric tester. The effects of Bi content on dielectric, ferroelectrc properties in SBT ceramics were discussed.

Abstract: SrBi2(Ta1-xNbx)2O9(SBTN) ferroelectric ceramics with typical bismuth layered perovskite structure were synthesized with a reactive hot-press sintering process at 1000°C for 2h. The as-sintered SBTN (x=0.2, 0.4, 0.6, 0.8) materials had a maximum relative density of 95.97%. Bi-layered perovskite structure was obtained. The grain sizes increased with increasing Nb content. In 1MHz frequency, remanent polarization (Pr) of SBTN (x=0.2, 0.4, 0.6, 0.8) were 10.0, 3.0, 1.7 and 1.0, while coercive field (Ec) of SBTN (x=0.2, 0.4, 0.6, 0.8) were 50, 32, 33 and 33kV/cm, respectively.

Abstract: Thermal analysis and Raman spectra were carried out in SrBi2Ta2O9 (SBT) nanoparticles to investigate phase transitions. Two anomalies have been observed in temperature dependence of specific heat for SBT nanoparticles. Under the combination with Raman spectra, it indicates that there exists a new ferroelectric intermediate phase in the phase-transition sequence. So we can conclude that the phase-transition sequence in SBT nanoparticles should be ferroelectric-ferroelectric-paraelectric. Moreover, the size effect was discussed in consideration of inner compressive stress in nanoparticles for this special transition behavior. The calculated results show that the SBT nanoparticles keep the ferroelectricity until the particle size is decreased to 4.2 nm.