Voronoi polyhedra analysis of MD simulated silicate glasses

The relationship between the volume and the surface of Voronoi polyhedra (VP) appeared to be an effective tool for the characterization of the structure of MD simulated silicate glasses. This feature was demonstrated on the series of binary potassium-silicate glasses with the alkali content ranging from 5 to 20 mol%. Moreover, the temperature dependence of the slope of the above dependence, when expressed for various types of central atoms (Si, O and K in our case) was studied. Each type of central atom is characterised by its own iron line linearly relating the volume of VP to its properly powered surface, so that new geometrical constrains to the VP was found.     

Relaxation of Alkali Glass Exposed to an Electron Beam

Binary potassium and rubidium silicate glasses were irradiated with high-energy electrons. The changes induced in the glasses were observed as a decay of alkali X-ray signals with time. The decay curves displayed incubation periods, connected with changes of the transport mechanism. The electron exposures were interrupted relative to the incubation periods, next the glasses were annealed to enable easier relaxations, and successively irradiated again to finish the decay curve. Relaxation, which depends on the point of the decay curve at which exposure is interrupted, is calculated for the glasses. Relaxation was found to depend strongly on the interruption point with regard to the incubation period. The longer irradiation causes worse recovery of the structure during annealing, especially if the irradiation lasts longer than the incubation time. The larger ions and lower concentration also inhibit relaxation.     

Configurational Entropy Relaxation of Silica Glass—Molecular Dynamics Simulations

Vitreous silica was modelled using molecular dynamics (MD). The glass structure was transferred into an undirected graph and decomposed into disjoint structural units that were ideally mixed to calculate the configurational entropy. The Debye relaxation model was suggested to simulate the evolution of entropy during the cooling of the system. It was found that the relaxation of the configurational entropy of MD corresponds to the effective cooling rate of 6.3 × 10⁶ Ks⁻¹ and its extrapolation to 0.33 Ks⁻¹ mimics the glass transition with T_g; close to the experimental value. Debye relaxation correctly describes the observed MD evolution of configurational entropy and explains the existence of freezing-in temperature and the shape of the curve in the transition region.     

Dynamics of a transport mediating switch

We show that dynamics of a general toggle switch that mediates transport between cellular compartments, converges to a unique periodic orbit in two limiting cases. These results can be perturbed for a restricted, but biologically ubiquitous model. This robustness may be the reason why only the restricted switch appears to be operating in Nature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

ZSM-5 zeolite synthesis in the presence of a copper-phthalocyanine complex characterization by ESR and Xe NMR

In the presence of a copper phthalocyanine complex (CuPc), it is possible to synthesize ZSM-5 zeolite containing the complex, and the crystallinity is good, provided that the amount of CuPc trapped is lower than the number of channel intersections where it can be located without too much distortion. The crystallinity is retained after decomposition of the CuPc in a stream of air at 500 °C. The copper-ZSM-5 system is characterized by the combined application of xenon adsorption, ESR and ¹²⁹Xe NMR spectroscopy.     

Formation and characteristics of graded-index optical waveguides buried in glass

Low-loss optical waveguides have been prepared through thermally induced ion exchange, where sodium ions in glass are replaced by silver ions from an unstirred silver nitrate melt. The index distributions of the inhomogeneous guides produced in the glass are deduced by analysing the measured mode spectra with the WKB-method. The increase in the refractive index caused by the presence of the silver is found to have a nearly Gaussian form with a peak of about 0.082. The index profile is correlated to the two parameters of the preparation process, the diffusion time and the operating temperature. In a typical case (9 hrs and 221°C) the index takes on its maximum value 2.6 μm below the surface of the glass and one obtains an 8-mode guide with five modes buried within the glass and three modes extending to the air-glass interface.A single-mode guide is formed in less than 5 min. The measurements yield an activation energy for the diffusion of Ag⁺ in glass of 8.4·10⁴J/mole and a diffusion coefficient at 221°C of 0.67·10⁻¹² cm²/s, in good agreement with the experimental results reported by other authors. The present observations suggest that a potential gradient is set up in the glass when silver diffuses into it so that the silver ions drift into the glass with a velocity of about 0.3μm/hr.