Correlation between the low temperature phonon mean free path and glass transition temperature in amorphous solids

An interpretation is offered for the increased chemical resistance of glass surfaces upon treatment with SiF₄/H₂O gaseous mixtures. Scanning electron microscopy examinations of the morphology and evolution of the surface structure, the surface sodium concentration profile as revealed by a nuclear reaction and ion release experiments support the formation of a very adherent amorphous silica layer - still transparent at the earlier stages of treatment - which fits well all experimental findings.     

Comparison of measurements of phase velocity in human calcaneus to Biot theory

Biot's theory for elastic propagation in porous media has previously been shown to be useful for modeling the dependence of phase velocity on porosity in bovine cancellous bone in vitro. In the present study, Biot's theory is applied to measurements of porosity-dependent phase velocity in 53 human calcanea in vitro. Porosity was measured using microcomputed tomography for some samples (n = 23) and estimated based on bone mineral densitometry for the remaining samples (n = 30). The phase velocity at 500 kHz was measured in a water tank using a through-transmission technique. Biot's theory performed well for the prediction of the dependence of sound speed on porosity. The trend was quasilinear, but both the theory and experiment show similar slight curvature. The root mean square error (RMSE) of predicted versus measured sound speed was 15.8 m/s.     

Stokes number effects in Lagrangian stochastic models of dispersed two-phase flows

The statistical properties of fluid velocities along particle trajectories in turbulent flows have a conditional dependency upon particle velocity. It is shown that the formulation of Lagrangian stochastic (LS) models for particle trajectories in terms of the well-mixed condition for these conditional velocity statistics is exactly analogous to the formulation of second-order LS models for fluid-particle trajectories. The particle aerodynamic response time is shown to be incorporated at second order, which together with the Lagrangian timescale introduced at first order, defines the Stokes number. Reynolds-number effects can be incorporated at third order. The corresponding Fokker-Planck equation is shown to be identical to that advocated by Pozorski and Minier [Phys. Rev. E 59 (1999) 855], who included the fluid velocities "seen" by a particle in the probability density function (pdf) formalism of Reeks and co-workers as a means of circumventing the closure problem (prescribing a closure for the particle flux induced by the fluid) associated with that approach. It is demonstrated that the neglect of Stokes-number effects accounts, in part, for the tendency of first-order LS models to underpredict particle deposition velocities in the diffusion-impaction regime.     

Superstatistical mechanics of tracer-particle motions in turbulence

The Lagrangian stochastic model of Reynolds [Phys. Fluids 15, L1-4 (2003)]] for the accelerations of fluid particles in turbulence is shown to predict precisely the observed Reynolds-number dependency of the distribution of Lagrangian accelerations and the exponents characterizing the observed extended self-similarity scaling of the Lagrangian velocity structure functions. Departures from superstatistics of the log-normal kind are accounted for and their impact upon model predictions is quantified.     

Synthesis of disaccharides related to the mycobacterial arabinogalactan

Several novel glycofuranoses disaccharides related to mycobacterial cell wall polysaccharides were synthesized regio- and stereoselectively using 2,3,5-tri-O-benzoyl-alpha-D-arabinofuranosyl trichloroacetimidate as a glycosyl donor.     

The structures of the α‐alums RbCr(SO4)2·12H2O and CsCr(SeO4)2·12H2O at 293 and 12 K

The crystal structures of the α‐alums rubidium chromium bis(­sulfate) dodecahydrate, RbCr(SO₄)₂·12H₂O, and caesium chromium bis[tetraoxoselenate(VI)] dodecahydrate, CsCr(SeO₄)₂·12H₂O, have been determined by X‐ray diffraction at 293 and 12 K. The metal atoms lie on sites and the anions lie on threefold rotation axes. The accurate and extensive data sets lead to much more precise determinations than are available from earlier work, particularly at 12 K. The changes in the atomic displacement parameters between 293 and 12 K correspond to the respective predominances of intermolecular and intramolecular vibrational effects.     

Monte Carlo study of electron correlation functions for small molecules

A study is made of electron-electron correlation functions for use in trial wave functions for small molecules. New forms are proposed that have only a few variational parameters, and these parameters have physical meanings that are easily discerned. Total energies for H₂, LiH and Li₂ computed using these correlation functions are presented, and comparison is made with previous forms, including the Jastrow-Pade form often used in Monte Carlo studies. We further treat the possibility that correlation depends not only on the separation of a pair of electrons but also on the location of the electron pair relative to the nuclei — indicative of a density-dependent or many body correlation effect. Our results indicate that such a many-body correlation effect is weakly present.This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098     

An improved set of mndo parameters for sulfur

The MNDO parameters for sulfur have been reoptimized. Calculations for a number of sulfur compounds indicate a very significant improvement. Inclusion of d AOs failed to correct the errors for compounds of sulfur in its higher valence states. Since d AOs are not included, the calculations are still confined to compounds of divalent sulfur.