Rare Earth Pyrophosphates: Effective Catalysts for the Production of Acrolein from Vapor-phase Dehydration of Glycerol

Vapor-phase dehydration of glycerol to produce acrolein was investigated at 320 A degrees C over rare earth (including La, Ce, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb, Lu) pyrophosphates, which were prepared by precipitation method. The most promising catalysts were characterized by means of XRD, FT-IR, TG-DTA, BET and NH3-TPD measurements. The excellent catalytic performance of rare earth pyrophosphate depends on the appropriate surface acidity which can be obtained by the control of pH value in the precipitation and the calcination temperature, e.g. Nd-4(P2O7)(3) precipitated at pH = 6 and calcined at 500 A degrees C in the catalyst preparation.

Vapor phase beckmann rearrangement of cyclohexanone oxime over rare earth pyrophosphates

A new application of rare earth pyrophosphates in vapor phase Beckmann rearrangement of cyclohexanone oxime was investigated. The rare earth phosphates were characterized by means of XRD, FT-IR, NH3-TPD and water contact angle measurement. It was found that the weak surface acidity and appropriate surface hydrophobicity should be two key factors in the excellent performance of these catalysts.

Preparation of methyl tert-butyl ether (MTBE) over heteropoly acids immobilized on activated carbon (HPA/C) in the vapor phase

Heteropolyacids (HPAs) supported on the activated carbon (SiW12/C and PW12/C) have been used to study the formation of methyl tert-butyl ether (MTBE). Compared to the conventional commercial catalysts, Amberlyst-15 resin and HZSM-5, HPAs supported catalysts have been proved to have much higher catalytic activity under lower temperature, especially selectivity to MTBE is up to 100%. It may be due to the high acid strength of HPAs as well as the specialty of heteropolyanion.

Vapor phase esterification catalyzed by immobilized dodecatungstosilicic acid (SiW12) on activated carbon

The vapor phase esterification of acetic acid with ethanol and n-butanol catalyzed by SiW12 supported on activated carbon was studied in a flow fixed-bed reactor in the range of 358 to 433 K. The effects of the reaction temperature, liquid hourly space velocity (LHSV) as well as the molar ratio on the catalytic activity have been investigated. The kinetic studies showed that the rate of esterification was dependent on the partial pressures of the reactants and the addition of argon, an inert diluent in the system when the total pressure was kept at 1 atm. Also the alcohol structure has a profound effect on not only the rate of esterification, but also on the mechanism of esterification changing from a dual site mechanism for ethanol to a single site mechanism for n-butanol.

Annealing behaviors of long-wavelength InAs/GaAs quantum dots with different growth procedures by metalorganic chemical vapor deposition

The effects of annealing on the optical properties of InAs/GaAs quantum dots (QDs) grown under different conditions by metalorganic chemical vapor deposition (MOCVD) are studied. A lower QD growth rate leads to an earlier and faster decrease of QD photoluminescence (PL) intensity with increasing annealing temperature. which is proposed to be related to the increased QD two-dimensional (2D)-three-dimensional (3D) transition critical layer thickness at low QD growth rate. High-quality GaAs cap layers grown at high temperature and a low deposition rate are shown to decrease the blueshift of the QDs' emission wavelength significantly during in-situ I h annealing experiments, which is important for the fabrication of long-wavelength InAs/GaAs QD lasers by MOCVD technique. (C) 2009 Elsevier B.V. All rights reserved.

Growth of InAs quantum dots on vicinal GaAs (100) substrates by metalorganic chemical vapor deposition and their optical properties

The growth of InAs quantum dots on vicinal GaAs (100) Substrates was systematically studied using low-pressure metalorganic chemical vapor deposition (MOCVD). The dots showed a clear bimodal size distribution on vicinal substrates. The way of evolution of this bimodal size distribution was studied as a function of growth temperature, InAs layer thickness and InAs deposition rate. The optical properties of dots grown on vicinal substrates were also studied by photoluminescence (PL). It was found that, compared with dots on exact substrates, dots on vicinal substrates had better optical properties such as a narrower PL line width, a longer emission wavelength, and a larger PL intensity. (c) 2006 Elsevier B.V. All rights reserved.

Performance of a Supersonic Model Combustor using Vaporized Kerosene Injection

Characteristics of supersonic combustion by injecting kerosene vapor into a Mach 2.5 crossflow at various preheat temperatures and pressures were investigated experimentally. A two-stage heating system has been designed and tested, which can prepare heated kerosene of 0.8 kg up to 820 K at pressure of 5.5 Mpa with minimum/negligible fuel coking. In order to simulate the thermophysical properties of kerosene over a wide range of thermodynamic conditions, a three-component surrogate that matches the compound class of the parent fuel was employed. The flow rate of kerosene vapor was calibrated using a sonic nozzle. Computed flow rates using the surrogate fuel are in agreement with the experimental data. Kerosene jets at various preheat temperatures injecting into both quiescent environment and Mach 2.5 crossflow were visualized. It was found that at injection pressure of 4 Mpa and preheat temperature of 550 K the kerosene jet was completely in vapor phase, while keeping almost the same penetration depth as compared to the liquid kerosene injection. Supersonic combustion tests were also carried out to compare the combustor performance for the cases of vaporized kerosene injection, liquid kerosene injection, and effervescent atomization with hydrogen barbotage, under the similar stagnation conditions. Experimental results demonstrated that the use of vaporized kerosene injection leads to better combustor performance. Further parametric study on vaporized kerosene injection in a supersonic model combustor is needed to assess the combustion efficiency as well as to identify the controlling mechanism for the overall combustion enhancement.

气相平衡扩散法制备β-BBO薄膜

用气相平衡扩散（VTE）法在α-偏硼酸钡单晶衬底上成功地制备出β-偏硼酸钡薄膜。使用X射线衍射技术（XRD）测试了样品的衍射峰，并比较了不同温度、不同保温时间、不同衬底方向对薄膜的影响。

Influence of AlN buffer layer thickness on structural properties of GaN epilayer grown on Si (111) substrate with AlGaN interlayer

We present the growth of GaN epilayer on Si (111) substrate with a single AlGaN interlayer sandwiched between the GaN epilayer and AlN buffer layer by using the metalorganic chemical vapour deposition. The influence of the AlN buffer layer thickness on structural properties of the GaN epilayer has been investigated by scanning electron microscopy, atomic force microscopy, optical microscopy and high-resolution x-ray diffraction. It is found that an AlN buffer layer with the appropriate thickness plays an important role in increasing compressive strain and improving crystal quality during the growth of AlGaN interlayer, which can introduce a more compressive strain into the subsequent grown GaN layer, and reduce the crack density and threading dislocation density in GaN film.

Effects of AlGaN/AlN Stacked Interlayers on GaN Growth on Si (111)

We report the growth of high quality and crack-free GaN film on Si (111) substrate using Al0.2Ga0.8N/AlN stacked interlayers. Compared with the previously used single AlN interlayer, the AlGaN/AlN stacked interlayers can more effectively reduce the tensile stress inside the GaN layer. The cross-sectional TEM image reveals the bending and annihilation of threading dislocations (TDs) in the overgrown GaN film which leads to a decrease of TD density.

Enhanced electroluminescence intensity of InGaN/GaN multi-quantum-wells based on Mg-doped GaN annealed in O-2

InGaN/GaN multi-quantum-well blue (461 +/- 4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 C in O-2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg-H complex by forming H2O, annealing in O-2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated Mg-Ga-O-N complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen. (C) 2008 American Institute of Physics.

Fabrication and excitation-power-density-dependent micro-photoluminescence of hexagonal nanopillars with a single InGaAs/GaAs quantum well

Hexagonal nanopillars with a single InGaAs/GaAs quantum well (QW) were fabricated on a GaAs (111) B substrate by selective-area metal-organic vapor phase epitaxy. The standard deviations in diameter and height of the nanopillars are about 2% and 5%, respectively. Zincblende structure and rotation twins were identified in both the GaAs and the InGaAs layers by electron diffraction. The excitation-power-density-dependent micro-photoluminescence (mu-PL) of the nanopillars was measured at 4.2, 50, 100 and 150 K. It was shown that, with increasing excitation power density, the mu-PL peak's positions shift to a higher energy, and their intensity and width increase, which were rationalized using a model that includes the effects of piezoelectricity, photon-screening and band-filling. It was also revealed that the rotation twins significantly reduce the diffusion length of the carriers in the nanopillars, compared to that in the regular semiconductors.

Using different carrier gases to control AlN film stress and the effect on morphology, structural properties and optical properties

On the metalorganic chemical vapour deposition growth of AlN, by adjusting H-2+N-2 mixture gas components, we can gradually control island dimension. During the Volmer - Weber growth, the 2-dimensional coalescence of the islands induces an intrinsic tensile stress. Then, this process can control the in-plane stress: with the N-2 content increasing from 0 to 3 slm, the in-plane stress gradually changes from 1.5 GPa tensile stress to - 1.2GPa compressive stress. Especially, with the 0.5 slm N-2 + 2.5 slm H-2 mixture gas, the in-plane stress is only 0.1 GPa, which is close to the complete relaxation state. Under this condition, this sample has good crystal and optical qualities.

Photoluminescence study of AlGaInP/GaInP quantum well intermixing induced by zinc impurity diffusion

AlGaInP/GaInP quantum well intermixing phenomena induced by Zn impurity diffusion at 540 degrees C were studied using room-temperature photo luminescence (PL) spectroscopy. As the diffusion time increased from 40 to 120 min, PL blue shift taken on the AlGaInP/GaInP quantum well regions increased from 36.3 to 171.6 meV. Moreover, when the diffusion time was equal to or above 60 min, it was observed firstly that a PL red shift occurred with a PL blue shift on the samples. After detailed analysis, it was found that the red-shift PL spectra were measured on the Ga0.51In0.49P buffer layer of the samples, and the mechanism of the PL red shift and the PL blue shift were studied qualitatively. (C) 2007 Elsevier B.V. All rights reserved.

Characterization of Thick GaN Films Directly Grown on Wet-Etching Patterned Sapphire by HVPE

Thick GaN films of high quality are directly grown on wet-etching patterned sapphire in a vertical hydride vapour phase epitaxy reactor. The optical and structural properties of GaN films are studied using scanning electronic microscopy and cathodoluminescence. Test results show that initial growth of hydride vapour phase epitaxy GaN occurs not only on the mesas but also on the two asymmetric sidewalls of the V-shaped grooves without selectivity. After the two-step coalescence near the interface, the GaN films near the surface keep on growing along the direction perpendicular to the long sidewall. Based on Raman results, GaN of the coalescence region in the grooves has the maximum residual stress and poor crystalline quality over the whole GaN film, and the coalescence process can release the stress. Therefore, stress-free thick GaN films are prepared with smooth and crack-free surfaces by this particular growth mode on wet-etching patterned sapphire substrates.