Microwave assisted heterogeneous catalysis: effects of varying oxygen concentrations on the oxidative coupling of methane

The oxidative coupling of methane was investigated over alumina supported La2O3/CeO2 catalysts under microwave dielectric heating conditions at different oxygen concentrations. It was observed that, at a given temperature using microwave heating, selectivities for both ethane and ethylene were notably higher when oxygen was absent than that in oxygen/methane mixtures. The differences were attributed to the localised heating of microwave radiation resulting in temperature inhomogeneity in the catalyst bed. A simplified model was used to estimate the temperature inhomogeneity; the temperature at the centre of the catalyst bed was 85 °C greater than that at the periphery when the catalyst was heated by microwaves in a gas mixture with an oxygen concentration of 12.5% (v/v), and the temperature difference was estimated to be 168 °C in the absence of oxygen

Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators

Three-dimensional (3D) topological insulators (TI) are novel quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The spin-helical Dirac surface states are expected to host exotic topological quantum effects and find applications in spintronics and quantum computation. The experimental realization of these ideas requires fabrication of versatile devices based on bulk-insulating TIs with tunable surface states. The main challenge facing the current TI materials exemplified by Bi2Se3 and Bi2Te3 is the significant bulk conduction, which remains unsolved despite extensive efforts involving nanostructuring, chemical doping and electrical gating. Here we report a novel approach for engineering the band structure of TIs by molecular beam epitaxy (MBE) growth of (Bi1-xSbx)2Te3 ternary compounds. Angle-resolved photoemission spectroscopy (ARPES) and transport measurements show that the topological surface states exist over the entire composition range of (Bi1-xSbx)2Te3 (x = 0 to 1), indicating the robustness of bulk Z2 topology. Most remarkably, the systematic band engineering leads to ideal TIs with truly insulating bulk and tunable surface state across the Dirac point that behave like one quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new TI devices with well-established semiconductor technology.Comment: Minor changes in title, text and figures. Supplementary information adde

X-ray absorption by the low-redshift intergalactic medium: A numerical study of the Lambda cold dark matter model

Using a hydrodynamic simulation of a cold dark matter universe with a cosmological constant, we investigate the X-ray forest absorption imprinted on the spectra of background quasars by the intervening intergalactic medium (IGM), at redshift z 0. In agreement with previous studies, we find that O VII and O VIII produce the strongest absorption features. The strong oxygen absorbers that might be detectable with Chandra or XMM-Newton arise in gas with T ~ 105.5-106.5 K and overdensities δ 100 that are characteristic of galaxy groups. Future X-ray missions could detect weaker oxygen absorption produced by gas with a wider range of temperatures and the lower densities of unvirialized structures; they could also detect X-ray forest absorption by carbon, nitrogen, neon, iron, and possibly silicon. If the IGM metallicity is Z = 0.1 Z, as we assume in most of our calculations, then the predicted number of systems strong enough for a ~5 σ detection with Chandra or XMM-Newton is extremely low. However, scatter in metallicity increases the number of strong absorbers even if the mean metallicity remains the same, making the predictions somewhat more optimistic. Our simulation reproduces the high observed incidence of O VI (λλ1032, 1038) absorbers, and the most promising strategy for finding the X-ray forest is to search at the redshifts of known O VI systems, thus reducing the signal-to-noise ratio threshold required for a significant detection. However, while many O VI absorbers have associated O VII or O VIII absorption, the O VI systems trace only the low-temperature phases of the X-ray forest, and a full accounting of the strong O VII and O VIII systems will require a mission with the anticipated capabilities of Constellation-X. The large effective area of the XEUS satellite would make it an extremely powerful instrument for studying the IGM, measuring X-ray forest absorption by a variety of elements, and revealing the shock-heated filaments that may be an important reservoir of cosmic baryons

Cathodoluminescence from interband transitions in germanium (111) and gallium arsenide (100) crystals

The cathodoluminescence spectra in n-type Ge(111) and semi-insulating (SI) GaAs(100) were measured in the range 2.20-5.20 eV. We observed five structures at 3.05, 3.22, 3.60, 3.90, and 4.30 eV in n-type germanium which are assigned to interband transitions. These results are similar to those of previous works on p-type Ge(111). For SI GaAs, the five structures observed at 2.95, 3.26, 3.88, 4.28, and 4.96 eV also indicate electron-hole recombination transitions between bands. All these results agree with the predictions of theoretical calculations. For lightly doped germanium, it is observed that the band structure does not depend on doping type. © 1995 The American Physical Society.published_or_final_versio

PS-DInSAR公共主影像的优化选取

Author name used in this publication: 陈强Author name used in this publication: 丁晓利, DING Xiao-liAuthor name used in this publication: 刘国祥Title in Traditional Chinese: PS-DInSAR公共主影像的優化選取Journal title in Traditional Chinese: 測繪學報2007-2008 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe