Tiecheng Lu

Transparent Ce:Y3Al5O12 ceramic phosphors for white light-emitting diodes

We present our recent achievement of a transparent ceramic able to produce white light when directly combined with commercially available blue light emitting diodes. The photoluminescence properties of ceramic phosphor (Y1-xCex)(3)Al5O12 are studied as a function of doping fraction (x = 0.0005-0.0020). The emission color is tunable by variations of Ce3+ concentration and ceramic phosphor thickness. A maximum luminous efficacy exceeding 93 lm/W at a low correlated color temperature of similar to 4600 K is obtained, which is superior to samples made from commercial phosphor powders. Hence, the present transparent ceramic phosphor is expected to be an ideal candidate for generating white light

Low-temperature high-pressure preparation of transparent nanocrystalline MgAl2O4 ceramics

Transparent MgAl2O4 spinel nanoceramics have been sintered at relatively low-temperature (500-700 degrees C) under high pressure (2-5 GPa) using a hydrostatic press with high-temperature-calcined nanopowders. The morphology, nanostructure, optical property, and density of ceramics were analyzed by scanning electron microscopy, transmission electron microscopy, UV-VIS-IR transmission spectrum, and Archimedes draining method. The average grain size (< 100 nm for all samples sintered) depends on the sintering pressure and temperature. The nanoceramics are highly transparent even though their relative densities are all less than 99%, due to the low or negligible light scattering from the nanosized grains and pores. (c) 2006 American Institute of Physics.

Related mechanism of transparency in MgAl(2)O(4) nano-ceramics prepared by sintering under high pressure and low temperature

We fabricate transparent MgAl(2)O(4) nano-ceramics, which are composed of 40 nm grains, by sintering under high pressure and low temperatures. Analysis of the grain size, differential strains, yield strength and porosity obtained from transmission electron microscopy and x-ray diffraction indicates that the pores at the grain boundary triple junctions can retard grain boundary migration and thus prevent grain growth. It is found that the relatively high density for MgAl(2)O(4) nano-ceramics produced at low-temperature and high pressure is attributed mainly to the large energy in the grain exteriors. The decrease in the transparency with increasing temperature (> 700 degrees C) is therefore a result of the light scattering at large pores. On the basis of these results, we propose a mechanism of transparency in MgAl(2)O(4) nano-ceramics.

Effect of electron irradiation on the transformation characteristics of narrow hysteresis TiNiCu shape memory alloys

TiNiCu shape memory alloy samples were irradiated by 1.7 MeV electrons below the martensite finish temperature Mf. The transformation temperatures and the latent heat of phase transformation were measured by differential scanning calorimeter. The damage accumulation was determined by positron annihilation technology. The results indicated that the austenite transformation temperatures were raised, and the hysteresis was increased by the irradiation. The electron irradiation had a slight effect on Mf, and no detectable effect on the martensitic transformation start temperature Ms. The second lifetime of positrons were increased by the electron irradiation indicating the increase in the size and amount of vacancy clusters, which contributed to the observed change of the transformation characteristics.

Defects in the reduced rutile single crystal

In this paper, the UV-VIS optical absorption spectra of oxidized and reduced rutile single crystals are measured by means of spectrophotometer and two absorption peaks around 430 and 730nm are found. These spectral data are analyzed by using the crystal field theory. Based on these studies, we suggest that the reduced crystal contain the defect center [Ti3+-O-v], with the oxygen vacancy (O-v) on one of the nearest neighbor sites of the central Ti3+ ion

A study on the effect factors of sol-gel synthesis of yttrium aluminum garnet nanopowders

Yttrium aluminum garnet (YAG) nanopowders were synthesized by sol-gel method using aluminum nitrate, yttrium nitrate, and citric acid as starting materials, de-ionized water, ethanol, and ethylene glycol as solvents, respectively. The phase formation process, state of particle size distribution (PSD), compositions, morphological characteristics, and thermal behavior of the powders were investigated by means of x-ray diffractometry, PSD, Fourier transform infrared, transmission electronic microscope, and thermogravimetry-differential scanning calorimetry. Results indicate that the formation and characteristics of precursor gel and YAG powder, such as the rate of gelation, average particle size, and powder agglomerate state, strongly depend on the stoichiometric amount of citric acid, the solvent composition, and the precalcination process. Highly crystalline, well-dispersed YAG nanopowder was obtained by calcining at 800 degrees C for 2 h in the presence of citric acid to nitrate ratio of 3, ethanol solvent, and precalcination process. According to the analysis of experimental results, sol-gel chemistry, DLVO theory, and steric effect, the effects of stoichiometric amount of citric acid, solvent composition, and precalcination process on the formation and characteristics of precursor gel and YAG powder have been discussed. Meanwhile, the overall synthesis mechanism in sol-gel method has been suggested.

Structure and Spatial Distribution of Ge Nanocrystals Subjected to Fast Neutron Irradiation

The influence of fast neutron irradiation on the structure and spatial distribution of Ge nanocrystals (NC) embedded in an amorphous SiO2 matrix has been studied. The investigation was conducted by means of laser Raman Scattering (RS), High Resolution Transmission Electron Microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). The irradiation of Ge-NC samples by a high dose of fast neutrons lead to a partial destruction of the nanocrystals. Full reconstruction of crystallinity was achieved after annealing the radiation damage at 8000C, which resulted in full restoration of the RS spectrum. HR-TEM images show, however, that the spatial distributions of Ge-NC changed as a result of irradiation and annealing. A sharp decrease in NC distribution towards the SiO2 surface has been observed. This was accompanied by XPS detection of Ge oxides and elemental Ge within both the surface and subsurface region.

Nanostructured VO2 film with high transparency and enhanced switching ratio in THz range

We investigated the terahertz (THz) transmission characteristics of semiconductor VO2 film and its THz suppression behavior after the phase transition. The VO2 films were deposited by the sol-gel method, and an in situ growth with surface nanocrystallization occurring in the films with increasing thickness was presented. Morphology-induced percolation leads to high THz transparency in the semiconductor VO2 film, and the more compact nanostructure could account for the enhanced THz switching ratio in the metallic film. These results may offer insights into the artificial design of VO2 films for THz device applications.

Ab initio many-body study of the electronic and optical properties of MgAl2O4 spinel

The electronic structure and optical properties of MgAl2O4 spinel with and without oxygen vacancies have been studied in the framework of many-body perturbation theory. By considering the self-energy of electrons, we reasonably describe the bandgap of perfect MgAl2O4 and the defect energy levels of MgAl2O4 containing oxygen vacancies. With the inclusion of electron-hole interaction by solving Bethe-Salpeter equation, the calculated dielectric functions and reflectivity spectrum all are in agreement well with the experimental results for perfect MgAl2O4. Our results show that the sharp peak near 7.8 eV in the experimental absorption spectrum is attributed to the excitonic states. The oxygen vacancies produce some new defect energy levels in the forbidden gap. The optical absorption peaks at 5.3 eV, 4.75 eV and 3.2 eV are induced by the V-O(0) and V-O(1+) vacancies

Novel phenomenon on valence unvariation of doping ion in Yb:YAG transparent ceramics using MgO additives

Yb:YAG nanopowders were synthesized by the alcohol-water co-precipitation method adding MgO as sintering additives. Appropriate amount of MgO adding can restrict the agglomeration and reduce the particle size of Yb:YAG powders. When the MgO content was 0.04wt%, well-dispersed Yb:YAG powders with ellipsoidal particles of less than 100 nm diameter were obtained. The experimental results showed the valence variation of doping ion Yb3+ would not appear when adding MgO as sintering additives, so ceramics showed colorless transparent instead of green due to Yb2+ color center using traditional SiO2 as additives. The transmission of the sintered Yb:YAG ceramics can reach 80.6% even without annealing. Ceramic morphology showed that the grains had uniform-distribution with the size of 10 μm or so, and no impurity and pore existed in the grain boundary and crystalline while using optimal sintering conditions.