Application of sol-gel process to H-zeolite synthesis over amorphous aluminosilicate materials

The impregnation preparation of H-zeolite over amorphous aluminosilicate solids (cracking catalysts) is compared with new synthesis procedures based on sol-gel process. The characterization results show that the H-zeolite reacts with the intermediates of the amorphous aluminosilicate.

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Nonlinear two-point boundary value problems with multiple solutions

In the first part of this paper we study the convergence of finite difference methods to approximate the maximal solution of problems of the form:u+f (x, u)=0, with boundary conditions eitheru(0)=u(b)=0 oru(0)=u (b)=0, 0<b1. The functionf(x, u) satisfies several conditions that are explicitly given in § 1. This work extends earlier results of Parter (see references at the end).Since this problem has in general more than one solution we develop in the second part two algorithms to approximate solutions characterized by the number of their zeros in (0, 1). We include in the last Section numerical results and some additional comments on the implementation of the algorithms on a digital computer.The research of this author was supported by the Office of Naval Research under contract N^o N-0014-67-A-0128-004.Part of this research was carried on while this author was visiting the Mathematics Research Center, University of Wisconsin, during the summer of 1969.     

Adsorption of a multicomponent mixture of gases with multisite occupancy

In this paper, we present a study of the adsorption of multicomponent mixtures with multisite occupancy. The transfer-matrix technique is used to analyze the one-dimensional binary mixtures for interacting systems. A general expression for the characteristic polynomial is derived. Extension of the treatment to a higher dimension is also presented by using the mean-field Bragg-Williams approximation, transfer-matrix calculations, and Monte Carlo simulation. Partial and total isotherms are obtained for the dimer-trimer mixture where a different order appears.     

Wavelet Jensen–Shannon divergence as a tool for studying the dynamics of frequency band components in EEG epileptic seizures

We develop a quantitative method of analysis of EEG records. The method is based on the wavelet analysis of the record and on the capability of the Jensen–Shannon divergence (JSD) to identify dynamical changes in a time series. The JSD is a measure of distance between probability distributions. Therefore for its evaluation it is necessary to define a (time dependent) probability distribution along the record. We define this probability distribution from the wavelet decomposition of the associated time series. The wavelet JSD provides information about dynamical changes in the scales and can be considered a complementary methodology reported earlier [O.A. Rosso, S. Blanco, A. Rabinowicz, Signal Processing 86 (2003) 1275; O.A. Rosso, S. Blanco, J. Yordanova, V. Kolev, A. Figliola, M. Schürmann, E. Ba?ar, J. Neurosci. Methods 105 (2001) 65; O.A. Rosso, M.T. Martin, A. Figliola, K. Keller, A. Plastino, J. Neurosci. Methods 153 (2006) 163]. In the present study we have demonstrated it by analyzing EEG signal of tonic–clonic epileptic seizures applying the JSD method. The display of the JSD curves enables easy comparison of frequency band component dynamics. This would, in turn, promise easy and successful comparison of the EEG records from various scalp locations of the brain.     

Phase coexistence in binary mixtures in thin films with symmetric walls: model calculations for two- and three-dimensional Ising lattices

Binary mixtures (A, B) that undergo phase separation in the bulk are considered in thin film geometry, assuming that one of the components is preferentially attracted to one of the walls. We discuss the average profile of the order parameter {volume fraction φ(z) of one of the components} in the z-direction (perpendicular to the surfaces), paying attention to the lateral inhomogeneity of the thin film when the average volume fraction corresponds to a state inside the coexistence curve. We consider the situation where due to (short range) surface forces a second-order wetting transition would occur in semi-infinite geometry: in the thin film geometry, this transition is rounded off and its only remnant is a smooth increase of the adsorbed mass in the surface-enriched layer in the transition region. Monte Carlo calculations for nearest neighbor Ising square and simple cubic lattices are used to derive typical concentration profiles. In the two-dimensional case, also the kinetics of domain formation after the quench from a disordered state is considered, and it is shown that the typical concentration oscillations perpendicular to the wall (“surface directed spinodal decomposition”) do not occur, due to strong lateral fluctuations of the local position of the interface between the enrichment layer at the surface and the neighboring depleted region. Finally, also phase-separated states in thin films with competing walls (where one surface prefers A and the other prefers B) are briefly treated, and experimental applications are discussed.     

Negative conductance and sequential tunneling in amorphous silicon-silicon carbide double barrier devices

Negative conductance has been observed in the electronic transport perpendicular to hydrogenated amorphous silicon carbide/amorphous silicon double-barrier structures at low temperatures. Some devices present negative resistance even at room temperature. These results are consistent with a sequential tunneling phenomenon.     

Seed layer effect on morphological,structural, and optical properties of electrochemically grown ZnO nanowires over different SnO2:F/glass substrates

Journal of Solid State Electrochemistry - Transparent conductive oxide electrodes and specifically SnO2:F/glass are widely employed substrates in the preparation of optoelectronic devices. This...     

Annealing effects of highly homogeneous a-Si1−xCx:H

We report the effect of annealing on the properties of amorphous hydrogenated silicon carbide thin films. The samples were deposited onto different substrates by plasma enhanced chemical vapor deposition at temperatures between 300 and 350 °C. The gaseous mixture was formed by silane and methane, at the ‘silane starving plasma regime’, and diluted with hydrogen. Rutherford backscattering and Fourier transform infrared spectrometry were used, respectively, to determine the atomic composition and chemical bonds of the samples. The film’s structure was analyzed by means of X-ray absorption fine structure and X-ray diffraction. For temperatures higher than 600 °C, amorphous silicon carbide films annealed under inert atmosphere (Ar or N₂) clearly changed their structural and compositional properties due to carbon loss and oxidation, caused by the presence of some oxygen in the annealing system. At 1000 °C, crystallization of the films becomes evident but only stoichiometric films deposited on single crystalline Si[1 0 0] substrates presented epitaxial formation of SiC crystals, showing that the crystallization process is substrate dependent. Films annealed in high-vacuum also changed their structural properties for annealing temperatures higher than 600 °C, but no traces of oxidation were observed or variations in their silicon or carbon content. At 1200 °C the stoichiometric films are fully polycrystalline, showing the existence of only a SiC phase. The XANES signal of samples deposited onto different substrates and annealed under high-vacuum also show that crystallization is highly substrate dependent.     

Study of the adsorption of interacting dimers on square lattices by using a cluster-exact approximation

A theoretical approach, based on exact calculation of the partition function on finite rectangular clusters, is introduced to study the adsorption of interacting homonuclear dimers on square lattices. An efficient algorithm allows us to calculate the detailed structure of the configuration space for m=(k×l) clusters with m varying from 8 to 48. The adsorption process has been monitored by following thermodynamic properties such as coverage versus chemical potential, internal energy and specific heat of the adlayer, etc. The analytical results are compared with those in [Surf. Sci. 411 (1998) 294], which were obtained by using Monte Carlo simulation and finite-size scaling techniques. The theoretical adsorption isotherms and phase diagrams (critical temperature versus coverage) for both attractive and repulsive lateral interactions are in good qualitative agreement with the computational data. This agreement between simulated and theoretical results supports the validity of the cluster-exact approximation proposed in this paper.