Papers by Author: Wei Pan

Abstract: Ultraviolet photodetectors (PDs) based on low-dimensional (LD) gallium oxide nanofibers were synthesized and assembled by a low cost and scalable electrospinning method. Highly uniaxially aligned nanofibers were used to assemble photodetectors. Photoconductive investigations indicate that the prepared photodetectors (PDs) are highly sensitive to ultraviolet (UV) light. The prepared photodetectors have shown a high photosensitivity (10³), fast photoresponse, excellent stability, and reproducibility under the illumination of UV light 254 nm. These electrospun nanofibers have also shown a high transparency (<85%) in the visible light 400-700 nm range. The high transparency of these nanobelts demonstrates their use for invisible UV photosensors.

Abstract: Air-plasma-sprayed (APS) thermal barrier coatings (TBCs) were oxidized in air at different temperatures for 1000h and sequentially investigated by impedance spectroscopy (IS) and scanning electron microscopy (SEM). After oxidation at temperatures higher than 900°C, a thermally grown oxide (TGO) layer was formed at the bond coat/topcoat interface in TBCs. The impedance spectra of oxidized TBCs typically contains two relaxation processes that stem from the yttria-stabilized zirconia (YSZ) topcoat of TBCs and the TGO layer. The TGO resistivity that obtained by simulating the impedance spectra increased with the increasing of annealing temperature, demonstrating the growth and the densification of TGO layer.

Abstract: The first principle calculation as well as the Boltzmann transport calculation have been employed to study the high temperature electronic transport properties of pyrochlore La₂Zr₂O₇. Combing constant scattering time approximation and experiment data, the electronic thermal conductivity and electron concentration are calculated as a function of temperature. The electronic thermal conductivity is 2.6×10^-4 W/(m.s) at 1270K and 7.2×10^-3 W/(m.s) at 1770K. The electron concentration increase rapidly with when the temperature is above 1600K.

Abstract: Light scattering caused by pores detrimentally affects the optical transparency of transparent ceramics. Herein, Mie theory has been used to calculate the cross-section of pore scattering in transparent ceramics, and the influence of wavelength, pore size distribution and refractive index has been discussed in detail. For wavelength between 200 nm and 2000 nm, the scattering cross-section decreases with increasing wavelength, which means that pore scattering is more detrimental to short-wavelength transparency. With ZOLD function simulating the pore size distribution inside the ceramic, it has been found that the scattering is strongest when the most-probable diameter d_m equals the incident light wavelength λ. And FWHM (full width at half maximum) parameter a also affects the scattering cross-section. a between 0.003 and 0.7 is necessary for obtaining high optical transparency in visible wavelength range. The method presented in this work is available for the estimation of scattering effect in different kinds of materials, which may be useful for future design of high-transparency ceramics.

Abstract: La₂NiO_4+σ nanofibers exhibiting typical Ruddlesden–Popper structure (K₂NiO₄) were fabricated by a facile electrospinning method. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to analyze the structure, morphology and crystal process of the La₂NiO_4+σ nanofibers. For electrical properties measurement, uniaxially aligned nanofibers were directly collected and assembled into electrode. In our research, La₂NiO_4+σ phase forms above 873K with no impurity phase emerges during the thermal treatments. The nanofibers are smooth and uniform throughout the entire length and the grain is growing as calcination temperature increases. Furthmore, the La₂NiO_4+σ nanofibers own high mixed conductivity at 773K, laying good foundation for intermediate temperature solid oxide fuel cells application.

Abstract: The relationship between the microstructure and the conductivity of nanocrystallized oxygen ionic electrolytes has been received great interest since it provides guidelines for designing electrolytes with high performances which might find applications in fuel cells and oxygen sensors. Here, we present a strategy for controlling the calcination temperature to tune the crystallite size and ionic transport properties of solid electrolyte. Different crystallite sizes of Ce_0.8Gd_0.2O_2-δ (CGO) nanofiber electrolytes were prepared. As the average crystallite size decreased from 27 nm to 8 nm, the conductivity of the nanofibers increased by more than five times. An exceptionally high oxide ion conductivity of 0.023 S∙cm^-1 for the nanofibers was observed at 550°C. These insights into the effect of the crystallite size on the structure and the conductivity allow a better control of the electrical properties of solid electrolytes, which might foster their applications in electrochemical devices operable at lower temperatures.

Abstract: Abstract. ZnO nanoparticles with different morphology were synthesized through a one-step and low temperature hydrothermal method with different reaction time. The prepared ZnO nanoparticles have been used as photocatalysts for the degradation of methylene blue (MB) aqueous solution under UV irradiation to study the relationship between the morphology and photocatalytic performance. The phase, crystallographic structure and morphology of synthesised ZnO nanoparticles were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The photocatalytic activity of ZnO nanoparticles were carried out by UV-visible spectroscopy (UV-vis). SEM results showed that different particle sizes and morphologies of flower-like, elliptical-shape and rod-shapes were obtained at 60 °C for 1 h, 4 h, 8 h and 12 h, which promoted photodegradation of methylene blue (MB) aqueous solution under UV light irradiation. Especially, elliptical-shape ZnOnanoparticles with reaction time of 4 h were most efficient, and the degradation rate was up to 98.2% after 20 min UV irradiation.

Abstract: YAG and Yb:YAG transparent ceramic nanopowders for laser crystal were synthesized by microwave-assisted alkoxide hydrolysis precipitation method. The YAG and Yb:YAG nanopowders were characterized by differential thermal analysis and thermo gravimetric analysis (DTA-TG), infrared spectrum (IR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and photoluminescence (PL) spectrum. The results show that the crystallizing temperature of the precursor of YAG and Yb:YAG is about 930 °C. The pure phase YAG and Yb:YAG nanopowders can be obtained at microwave radiation power of 385 W, microwave radiation time of 30 min and sintering temperature of 1100 °C. The average particle size of YAG is about 120 nm. Yb:YAG is uniform sphere and the average sizes is about 100 nm. The intensity of luminescence of Yb:YAG nanopowders decreased with the increasing of microware time. The maximum luminous intensity of the Yb:YAG nanopowders can be obtained when the microwave radiation is 30 min.

Abstract: Diopside glass-ceramics were prepared by reactive crystallization with mixed powder of waste glass and mullite. The effects of mullite content and sintering temperature and holding time on the crystal phase and morphology of diopside glass-ceramics were investigated. The reactive crystallizing behavior between waste glass powder and mullite was studied using energy dispersive spectrdmeter during the sintering and holding process. The result show that the optimum preparation conditions were as follows: the addition of 30 wt% mullite used as crystallization promoting agent, sintering temperature of 900 °C and holding time of 2h. The main difference between reactive crystallization method and the traditional method of preparing diopside glass-ceramics was the reactive crystallization mechanism. The diopside and corundum crystals were formed via an interaction between glass powder and mullite instead of direct precipitation from the parent glass during the calcining and holding process. With the increasing of sintering temperature and holding time, the Ca and Mg element concents of the glass were increased, on the contrary, that Si element concent was decreased in the mullite. It shows that the Si element diffused from mullite to glass and precipitated diopside crystals.

Abstract: In this paper, a series of Al₂O₃/Ni composites with various content of Ni (0 vol%, 10 vol%, 20 vol%, 30 vol%, 40 vol% and 50 vol%) were prepared, and the effect of the methods and compositions on the density and electromagnetic properties was investigated. The results show that the density of the composites increases with the increasing sintering temperature as well as the increasing content of Ni. Moreover, the dielectric constant of the composites increases remarkably with the sintering temperature and the content of Ni, while the permeability shows an almost linear enhancement with the increase of Ni content. This suggests that the Al₂O₃/Ni composite can be expected to be a promising substrate material with certain electromagnetic compatibility.