Ab initio calculations of the equilibrium geometry of cis and trans methylglyoxal using the force method

Ab initio calculations of the equilibrium ground-state conformation of trans-methylglyoxal, CH₃COCHO, have been carried out using the gradient method and a 4-3 IG basis set. The results are compared with experiment and with a theoretical geometry computed using an STO-3G basis set, and the more usual stepwise optimization of the parameters. The molecular orbital energies, dipole moment and ionization energy are calulated. Calculations are also carried out on the cis-isomer and the barrier to cis-trans isomerization is obtained.     

Raman scattering and phase state of H:LiTaO3 waveguide thin films

Raman scattering of optical waveguides formed by proton-exchange in lithium tantalate is studied. The effect of a post-exchange annealing up to 420°C on the optical properties and Raman spectra are also investigated. Two types of Raman bands are observed: those which are very strongly influenced by the presence of protons (proton-related bands) and those characteristic of the pure crystal (bulk bands). The intensity of the two kinds of bands depends in an opposite way on the annealing temperature, with the proton-related bands following the anomalous behaviour of n _e. The spectra from samples annealed up to the temperature of the maximum value of n _e are remarkably similar to the high-temperature paraelectric spectra of a LiTaO₃ single crystal.     

Investigation of the effects of porous layer on the electrical properties of Pt/n-GaN Schottky contacts

This article reports the effects of porosity on the enhancement of the Schottky barrier height (SBH) and reduction of the leakage current of Pt Schottky diode based on n-type GaN. The porous GaN layer in this work was generated by electroless chemical etching under UV illumination. For comparative study, two additional samples, i.e. one as-grown sample and one thermally treated sample, were also prepared. The structural studies showed that the porous GaN sample has a relatively rough surface morphology with slightly broader X-ray diffraction peak of the (0 0 0 2) plane. The current-voltage (I-V) measurements revealed that the electrical characteristics of the Pt Schottky diode were improved significantly by the presence of the porous layer, in which SBH was observed to be enhanced by 27%.     

Structural characteristics of porous polymers treated by freezing with water or acetone

The pore structure of nine polymers of different origin initial and treated in water and acetone for 24 h and then frozen by liquid nitrogen for 2 h was studied using low-temperature nitrogen adsorption. An average increase in pore volume and surface area is approximately 16 and 8%, respectively, after freezing with water and 19 and 8%, respectively, after treatment with acetone. However, for some samples, this effect is negative but for others, structural changes are significantly greater (up to 106%) than the average one. Certain treated polymers are characterized by shifted pore size distribution which is also accompanied by displacement of nitrogen adsorption energy distribution.     

On the detection of trends in long-term correlated records

We use the Detrended Fluctuation Analysis (DFA) to quantify underlying trends in long-term correlated records. Our approach is based on the fact that different orders of DFA are affected differently by trends. For a given instrumental record of length N, we compare the fluctuation exponent α₀ of DFA0 where trends are not being eliminated, with the fluctuation exponent α₂ of DFA2 where possible linear trends in the instrumental record are being eliminated. From this we deduce numerically the probability density p(A) that in the considered long-term correlated record, a linear trend with a slope between A and occurs. Without loss of generality we focus on Gaussian distributed data. As an example, we apply our analysis to several long temperature records (Melbourne, Oxford, Prague, Pusan, Uppsala, and Vienna), where we discuss the trends within the last 90 years, which may originate from both, urban and global warming.     

Alkylation of imidazole under ultrasound irradiation over alkaline carbons

N-Alkyl-imidazole has been synthesized by sonochemical irradiation of imidazole and 1-bromobutane using alkaline-promoted carbons (exchanged with the binary combinations of Na, K and Cs). The catalysts were characterized by X-ray photoelectron spectroscopy, thermal analysis and N₂ adsorption isotherms. Under the experimental conditions, N-alkyl-imidazoles can be prepared with a high activity and selectivity. It is observed that imidazole conversion increases in parallel with increasing the basicity of the catalyst. The influence of the alkaline promoter, the reaction temperature, and the amount of catalyst on the catalytic activity has been studied. For comparison, the alkylation of imidazole has also been performed in a batch reactor system under thermal activation.     

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     

Measurement of Electrical Conductivity of Porous Titanium and Ti6A14V Prepared by the Powder Metallurgy Method

Porous titanium and Ti6A14V are produced by the powder metallurgy method. Dependence of the electrical conductivity on the porosity and pore size is investigated and the experimental results are correlative and compared with several earlier models. A newly modified Mori-Tanaka relationship based on the effective field method is proposed, which is successfully applied to describe the dependence of the electrical conductivity of porous titanium and Ti6A14V on the porosity. The pore size has a minor effect on the electrical conductivity of both samples.     

Development of porous metal oxide thin films by co-evaporation

This paper focuses on the development of mixed metal oxide thin films and physical characterization of the films. The films were produced by co-evaporation of titanium oxide and tungsten oxide powders. This allowed the development of titanium oxide-tungsten oxide films as analyzed using XPS. Examination in the SEM and AFM showed that the films were nanoporous with the pore size and pore orientation varying as a function of the deposition angle. UV-vis spectra of the films show an increase of transmittance with increasing deposition angle which is attributed to the structure and porosity of the films. Raman analysis indicated that the as-deposited films have broad and weak Raman characteristics, attributed to the nanocrystal nature of the films and the presence of defects, and the peak broadening deceases after annealing the film, as expected.