Non-linear viscoelastic behavior of fumed silica suspensions

Suspensions of fumed silica exhibit a wide range of rheological properties depending on the nature and magnitude of the interparticle forces. In a non-polar fluid, the particles interact through hydrogen bonding and can form a three-dimensional network. The microstructure formation is responsible for the non-linear viscoelastic behavior of fumed silica suspensions, even at very small strain. These non-linear rheological properties have been studied in small amplitude oscillatory experiments as a function of particle size, surface treatment of particles, suspending medium polarity and solids concentration. The non-linear viscoelastic behavior is characterized by a non-sinusoidal waveform of the signal response. For suspensions in a non-polar fluid, both the elastic and the loss moduli are shown to be sensitive to the strain amplitude: the elastic modulus is decreasing with increasing strain whereas the loss moduli is initially increasing with strain. We have chosen to examine the dissipated energy which is clearly related to the breakup of the suspension structure. A comparison of model predictions and the experimental data shows the limitations of these models, recently proposed in the literature to describe the behavior of colloidal suspensions. Received: 9 March 1998 Accepted: 17 November 1998     

A decomposed Crouzeix-Raviart element for the finite element solution of the Navier-Stokes equation

A methodology for the decomposition of the Crouzeix-Raviart finite element into six linear subelements is described. The resulting element is shown to satisfy the Brezzi-Babu?ka compatibility condition. The error bounds are also established. A comparison in accuracy between this and the standard Crouzeix-Raviart element is presented for driven cavity flows. Other results include the execution time for the DCR element and the Crouzeix-Raviart element along with both analytical and numerical integration. It is shown that the decomposed element results in shorter execution times with only marginal changes in accuracy.     

On-line monitoring of composite nanoparticles synthesized in a pre-industrial laser pyrolysis reactor using Laser-Induced Breakdown Spectroscopy

Laser-Induced Breakdown Spectroscopy (LIBS) was employed for on-line and real time process monitoring during nanoparticle production by laser pyrolysis. Laser pyrolysis has proved to be a reliable and versatile method for nanoparticle production. However, an on-line and real time monitoring system could greatly enhance the process optimization and accordingly improve its performances. For this purpose, experiments aiming at demonstrating the feasibility of an on-line monitoring system for silicon carbide nanoparticle production using the LIBS technique were carried out. Nanosecond laser pulses were focused into a cell through which part of the nanoparticle flux diverted from the production process was flowed for LIBS analysis purposes. The nanoparticles were vaporized within the laser-induced plasma created in argon used as background gas in the process. Temporally-resolved emission spectroscopy measurements were performed in order to monitor nanoparticle stoichiometry. Promising results were obtained and on-line Si/C_x stoichiometry was successfully observed. These results put forward the possibility of real time correction of the nanoparticle stoichiometry during the production process.     

Catalytic Asymmetric Mannich Reactions with Fluorinated Aromatic Ketones: Efficient Access to Chiral β‐Fluoroamines

Reported herein is a Zn/Prophenol‐catalyzed Mannich reaction using fluorinated aromatic ketones as nucleophilic partners for the direct enantio‐ and diastereoselective construction of β‐fluoroamine motifs featuring a fluorinated tetrasubstituted carbon. The reaction can be run on a gram scale with a low catalyst loading without impacting its efficiency. Moreover, a related aldol reaction was also developed. Together, these reactions provide a new approach for the preparation of pharmaceutically relevant products possessing tetrasubstituted C? F centers.     

Toward glasses with better indentation cracking resistance

The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass surface under sharp contact loading and the extent to which these cracks develop is intimately related to the way the material attempts to relax the corresponding stress field. Two processes which are known to occur upon indentation are densification and isochoric shear flow. The contributions of both mechanisms were quantitatively assessed for glasses belonging to different chemical systems in previous papers ,  and . In the present study, indentation cracking maps are provided, which offer guidelines to the design of glasses with better surface damage resistance based on their elastic properties and hardness.     

Atomic-scale simulation of intergranular segregation of H in Al–Mg: implications for H-induced damage

The goal of this work is to make a contribution to the understanding of the microscopic mechanisms of H-induced intergranular damage. We develop an embedded-atom method interatomic potential for H in the Al–Mg system with the main aim of reproducing the current understanding of H trapping to vacancies. This model is used to investigate the effect of the Mg–H affinity on the segregation of H on the Σ =5 (310) [001] grain boundary. Monte Carlo simulations in the grand canonical ensemble are used to estimate equilibrium H concentrations at this boundary at T=300?K. A large structure change, associated with the H enrichment of the grain boundary, is reported. The implications on damage to the interface are discussed.     

Cyclobutanes in catalysis

The exploitation of ring strain as a driving force to facilitate chemical reactions is a well‐appreciated principle in organic chemistry. The most prominent and most frequently used compound classes in this respect are oxiranes and cyclopropanes. For rather a long time, cyclobutanes lagged behind these three‐membered‐ring compounds in their development as reactive substrates, but during the past decade an increasing number of useful reactions of four‐membered‐ring substrates have emerged. This Minireview examines corresponding catalytic reactions ranging from Lewis or Brønsted acid catalyzed processes to enzymatic reactions. The main focus is placed on transition‐metal‐catalyzed C C bond‐insertion and β‐carbon‐elimination processes, which enable exciting downstream reactions that deliver versatile building blocks.