Global Wave Solution of generalized MHD Equations in a Cylindrical Plasma

The linear eigenstate problem of generalized magnetohydrodynamics(MHD) equations in a cylindrical plasma is discussed. The effects of finite frequency and finite pressure perturbation lead to an important result: the resonant layer of the shear Alfven waves is not a singular layer. In this paper, the MHD equations are reduced to four differential equations of first order for perturbed quantities. An analytical dispersion relation for a homogeneous plasma cylinder is obtained. The K. Appert theory is a limiting case of our theory     

Study of silica aerogel grain surfaces by using a positron age–momentum correlation technique

The effect of heat treatment on silica aerogel has been studied by a positron age–momentum correlation technique and infrared measurement. A difference was observed between the momentum distributions of the electrons on the first layers of the silica aerogel grain surfaces and the electrons in the grains in an as-supplied sample, but not in the sample heat treated at 800 °C. A large change in the S parameter for the momentum distribution of the electrons on the first layer occurs around 200 °C. This change correlates well with that of the infrared spectra, which show oxidation of the methoxyl groups at temperatures above around 200 °C. This correlation reveals that those groups are mainly located on the first layer of the silica grains. Received: 13 November 2000 / Accepted: 23 July 2001 / Published online: 30 October 2001     

Positron lifetime measurement in NaNO2

Positron lifetimes in NaNO₂ were measured as a function of temperature. The lifetime spectra consisted of one component, and could not be resolved into two component. The positron trapping model is not applicable in this case. Observed changes of lifetimes around the transition temperature could not be understood only by the volume dilatation, but they must be affected by the ferro-para electric phase transition     

Comparison of InGaN/GaN light emitting diodes grown on m ‐plane and a ‐plane bulk GaN substrates

InGaN/GaN‐based light emitting diodes (LEDs) grown on m ‐plane, a ‐plane and off‐axis between m ‐ and a ‐plane GaN bulk substrates were investigated. A smooth surface was obtained when a ‐plane substrate was applied; however, large amounts of defects were observed. Photoluminescence measurements of the LEDs with a well thickness of 2.5 nm revealed that all the LEDs showed the peak emission wavelength at 389 nm. The PL intensity of the a ‐plane LED is one order of magnitude lower than that of the m ‐plane LED. The a ‐plane LEDs showed significant lower electroluminescence output powers than m ‐plane LEDs, suggesting that excitons are trapped by the defects, which act as non‐radiative recombination centers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pulse trains propagating through excitable media subjected to external noise

We study the propagation of periodic pulse trains in excitable media exposed to external spatio-temporal noise using the light-sensitive Belousov-Zhabotinsky reaction with the underlying Oregonator model as representative example. In the weak noise approximation we find noise-induced transitions in the dispersion relation of pulse trains. We discuss noise-enhanced propagation of pulse trains within a certain wave-length range caused by external noise of moderate strength.     

Improved electroluminescence on nonpolar m ‐plane InGaN/GaN quantum wells LEDs

Improved nonpolar m ‐plane light emitting diodes (LEDs) with a thick InGaN multi‐quantum‐well (MQW) structure have been fabricated on low extended defect bulk m ‐plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence emission from the packaged LEDs was 402 nm, which is in the blue‐violet region. The output power and EQE were 28 mW and 45.4%, respectively, at a pulsed driving current of 20 mA. With increasing current, the output power increased linearly, and fairly flat EQE was observed with increasing drive current. At 200 mA, the power and EQE were 250 mW and 41%, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diode-pumped Nd:YVO4-Nd:CNGG crystals blue laser at 462 nm by intracavity sum-frequency-mixing

We report a laser architecture to obtain continuous-wave (cw) blue radiation at 462 nm. A 808 nm diode-pumped the Nd:YVO₄ crystal emitting at 914 nm. A part of the pump power was then absorbed by the Nd:YVO₄ crystal. The remaining was used to pump the Nd:CNGG crystal emitting at 935 nm. Intracavity sum-frequency mixing at 914 and 935 nm was then realized in a LiB₃O₅ (LBO) crystal to reach the blue radiation. We obtained a continuous-wave output power of 892 mW at 462 nm with a pump laser diode emitting 18.4 W at 808 nm.     

Unraveling the carbene chemistry of oxymethylene ethers: Experimental investigation and kinetic modeling of the high-temperature pyrolysis of OME-2

Oxymethylene ethers (OMEs) form an interesting family of synthetic compounds to replace fossil fuels. This alternative liquid energy carrier can contribute to a circular carbon economy when produced via carbon capture and utilization technology using renewable electricity. Despite the potential to reduce greenhouse gas and particulate matter emissions and their ideal ignition characteristics, little is known about the thermal decomposition behavior of OMEs. In this work, new insights are obtained in the pyrolysis chemistry of oxymethylene ether-2 (OME-2) and the role of carbenes by performing experiments at high temperatures (> 850 K) in a tubular quartz reactor. The used continuous bench-scale pyrolysis unit has a dedicated on-line analysis section including comprehensive two-dimensional gas chromatography (GC $\times$ GC) coupled with flame ionization detection (FID) and mass spectroscopy (MS) to identify and quantify the full product spectrum over the complete temperature range. The reactor temperature was varied between 850 and 1150 K at a fixed pressure of 0.15 MPa and residence times of 400 to 850 ms. The major products are dimethoxymethane, formaldehyde, methyl formate, methane, CO₂, CO and H₂. Minor intermediate compounds comprise dimethyl ether, formic anhydride, formic acid, methoxymethyl formate and methoxymethanol. The yields of compounds with carbon-carbon bonds are low since no such bonds originally occur in OME-2. Precursors of aromatic compounds and soot particles are absent in the reactor effluent. The experimental results are simulated with a new first principles-based kinetic model for pyrolysis and combustion of OME-2. This model can predict the experimental trends of major products on average within the experimental uncertainty margin of ± 10% relative for major product species. A reaction pathway and sensitivity analysis are presented to highlight the importance of the carbenes for initiation of the radical chemistry under pyrolysis conditions.