Silicon Carbide Modified Carbon Materials. Formation of Nanocrystalline SiC from Thermochemical Processes in the System Coal Tar Pitch/Poly(carbosilane)

Poly(carbosilane) or PCS, {–CH₂–SiH(CH₃)–} _n , is used as a Si-bearing precursor in combination with a coal tar pitch to study thermally induced transformations toward SiC-modified carbon composites. Following mixing of the components in the molten pitch at 160°C, the mixture is heated under argon atmosphere at 500°C yielding a solid carbonizate that is further subjected to separate pyrolysis experiments at 1300°C or 1650°C. At temperatures up to 500°C, the PCS reacts with suitable pitch components as well as undergoing decomposition reactions. At higher temperatures, clusters of prevailingly nanocrystalline -SiC are confirmed after the 1650°C pyrolysis step with indications that the formation of the compound starts at 1300°C. ²⁹Si MAS NMR, XRD, FT-IR, XPS, and elemental analysis are used to characterize each pyrolysis step, especially, from the viewpoint of transformation of silicon species to silicon carbide in the carbon matrix evolved from the pitch.     

On the superintegrability of TTW model

The superintegrability of so-called Tremblay–Turbiner–Winternitz (TTW) model has been conjectured on the basis of the fact that all its trajectories are closed. This conjecture has been proven using the method based on solving the partial differential equations for two functions having the same Poisson bracket with the Hamiltonian. In the present short Letter we show that superintegrability of TTW model can be established by using well-known elegant techniques of analytical mechanics. Moreover, the resulting expression (after an appropriate ordering) can be generalized to the quantum-mechanical case.     

Solubility of rare earth metal bromides and iodides in aqueous systems

Abstract  

The International Union of Pure and Applied Chemistry (IUPAC) project of collection, compilation, and critical evaluation of solubility data of bromides and iodides of the scandium group and all lanthanides in water and aqueous systems containing either halide acids, halide salts, or organic compounds is under preparation. As a result of their similarity to the chlorides, which were recently evaluated, the bromides and iodides in the lanthanide series should show some regularities in their solubility data. Unfortunately, the corresponding results show a large scatter when ordered according to the atomic number. Thus, it is complicated to select the best data for recommendation. Reasons for the inaccuracy of solubility measurements are outlined. In fact some solubility values of bromides predicted by correlation with chlorides seem to be more reliable than the experimental ones. As sufficient experimental data at various temperatures were available, the water-rich fragment of the LaBr₃–H₂O equilibrium phase diagram has been formed and depicted. It seems to be similar to the well-known LaCl₃–H₂O diagram. Several regularities, with respect to stoichiometry and solubility of compounds formed, were observed during investigations of the aqueous ternary systems. The complex iodides of various lanthanides display more regularities in their properties than the bromides do.     

Replica Exchange and Multicanonical Algorithms with the coarse-grained UNRES force field

Three algorithms, namely a Replica Exchange method (REM), a Replica Exchange Multicanonical method (REMUCA), and Replica Exchange Multicanonical with Replica Exchange (REMUCAREM), were implemented with the coarse-grained united-residue force field (UNRES) in both Monte Carlo and Molecular Dynamics versions. The MD algorithms use the constant-temperature Berendsen thermostat, with the velocity Verlet algorithm and variable time step. The algorithms were applied to one peptide (20 residues of Alanine with free ends; ala(20)) and two small proteins, namely an α-helical protein of 46 residues (the B-domain of the staphylococal protein A; 1BDD), and an α+β-protein of 48 residues (the E. Coli Mltd Lysm Domain; 1E0G). Calculated thermodynamic averages, such as canonical average energy and heat capacity, are in good agreement among all simulations for poly-L-alanine, showing that the algorithms were implemented correctly, and that all three algorithms are equally effective for small systems. For protein A, all algorithms performed reasonably well, although some variability in the calculated results was observed whereas, for a more complicated α+β-protein (1E0G), only Replica Exchange was capable of producing reliable statistics for calculating thermodynamic quantities. Finally, from the Replica Exchange molecular dynamics results, we calculated free energy maps as functions of RMSD and radius of gyration for different temperatures. The free energy calculations show correct folding behavior for poly-L-alanine and protein A while, for 1E0G, the native structure had the lowest free energy only at very low temperatures. Hence, the entropy contribution for 1E0G is larger than that for protein A at the same temperature. A larger contribution from entropy means that there are more accessible conformations at a given temperature, making it more difficult to obtain an efficient coverage of conformational space to obtain reliable thermodynamic properties. At the same temperature, ala(20) has the smallest entropy contribution, followed by protein A, and then by 1E0G.     

Technology aspects of GaN-based diodes for high-field operation

This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.     

Determination of 7-Ketocholesterol and 7-Hydroxycholesterol in Meat Samples by TLC with Densitometric Detection

JPC – Journal of Planar Chromatography – Modern TLC - Concentrations of two oxycholesterols: 7-ketocholesterol and 7- hydroxycholesterol in meat samples (raw and cooked) were determined...     

Thermodynamic and physicochemical properties of binary mixtures of nitromethane with {2-methoxyethanol + 2-butoxyethanol} systems at T = (293.15, 298.15, 303.15, 308.15, and 313.15) K

The density, relative permittivity, viscosity and speed of sound at T = (293.15, 298.15, 303.15, 308.15, and 313.15) K in the binary mixtures of nitromethane with 2-methoxyethanol and 2-butoxyethanol have been measured as a function of composition. From the experimental results, the excess molar volumes V^E, excess Gibbs free energy of activation for viscous flow $(\mathrm{\Delta }{G}^{1E})$, excess isentropic compressibility $({\kappa }_s^E)$ and the deviations in the relative permittivity, viscosity, and speed of sound from a mole fraction average have been calculated. The viscosity data, at T = 298.15 K, were correlated with equations of Hind et al., Grunberg and Nissan, Frenkel, and McAllister. The results are discussed in terms of intermolecular interactions and structure of studied binary mixtures.     

Highly effective synthesis of vinylfunctionalised cubic silsesquioxanes

A series of functionalised cubic monovinylheptaisobutylsilsesquioxanes and octavinylsilsesquioxanes were obtained via efficient and highly stereoselective cross-metathesis and silylative coupling with substituted styrenes.     

The uniqueness of endpoints for set-valued dynamical systems of contractions of Meir–Keeler type in uniform spaces

In this paper, the concept of the set-valued dynamical systems of contractions of Meir–Keeler type in uniform spaces is introduced and conditions guaranteeing the existence and uniqueness of endpoints of these contractions and the convergence to these endpoints of all generalized sequences of iterations of these contractions are established. The definition and the result presented here are new for set-valued dynamical systems in uniform, locally convex and metric spaces and even for single-valued maps. Examples show a fundamental difference between our result and the well-known ones.     

Numerical modeling of large deformations in soil structure interaction problems using FE,EFG, SPH,and MM-ALE formulations

Present design practice for soil structure interaction (SSI) problems most frequently assumes linear elastic properties of the soil and disregards geometrical nonlinearities, treating the displacements as small. However, there are numerous problems that require a more advanced approach. This paper presents an application of such numerical approaches to modeling SSI problems in the presence of large soil deformations. Simulations using Lagrangian finite element, element-free Galerkin, smoothed particle hydrodynamics (SPH), and multi-material arbitrary Lagrangian Eulerian (MM-ALE) approaches were performed for two previously conducted experimental tests: (1) large-scale steel pad penetration into silty clay with sand and (2) standard cone penetration test performed on poorly graded sand. In this paper, the usefulness and the efficiency of the methods was assessed in terms of modeling robustness and computational cost. Results show that to some extent each of the utilized methods is able to capture large deformations. However, the most robust turned out to be SPH and MM-ALE methods as the only two that were successful in simulating both experiments.