Enantioselective Redox-Divergent Chiral Phosphoric Acid Catalyzed Quinone Diels–Alder Reactions

An efficient enantioselective construction of tetrahydronaphthalene-1,4-diones as well as dihydronaphthalene-1,4-diols by a chiral phosphoric acid catalyzed quinone Diels–Alder reaction with dienecarbamates is reported. The nature of the protecting group on the diene is key to the success of achieving high enantioselectivity. The divergent “redox” selectivity is controlled by using an adequate amount of quinones. Reversible redox switching without erosion of enantioselectivity was possible from individual redox isomers.     

Nickel‐Catalyzed Amide Bond Formation from Methyl Esters

Despite being one of the most important and frequently run chemical reactions, the synthesis of amide bonds is accomplished primarily by wasteful methods that proceed by stoichiometric activation of one of the starting materials. We report a nickel‐catalyzed procedure that can enable diverse amides to be synthesized from abundant methyl ester starting materials, producing only volatile alcohol as a stoichiometric waste product. In contrast to acid‐ and base‐mediated amidations, the reaction is proposed to proceed by a neutral cross coupling‐type mechanism, opening up new opportunities for direct, efficient, chemoselective synthesis.     

A Fast Algorithm for Invasion Percolation

We present a computationally fast Invasion Percolation (IP) algorithm. IP is a numerical approach for generating realistic fluid distributions for quasi-static (i.e., slow) immiscible fluid invasion in porous media. The algorithm proposed here uses a binary-tree data structure to identify the site (pore) connected to the invasion cluster that is the next to be invaded. Gravity is included. Trapping is not explicitly treated in the numerical examples but can be added, for example, using a Hoshen–Kopelman algorithm. Computation time to percolation for a 3D system having $N$ total sites and $M$ invaded sites at percolation goes as $O(M \log M)$ for the proposed binary-tree algorithm and as $O(M N)$ for a standard implementation of IP that searches through all of the uninvaded sites at each step. The relation between $M$ and $N$ is $M = N^{D/E}$ , where $D$ is the fractal dimension of an infinite cluster and $E$ is Euclidean space dimension. In numerical practice, on finite-sized cubic lattices with invasion structures influenced by the injection boundary and boundary conditions lateral to the flow direction, we observe the scaling $M = N^{0.852}$ in 3D (valid through the second decimal place) instead of $M= N^{0.843}$ based on the infinite cluster fractal dimension $D=2.53$ .     

Quantum Hall conductivity in a Landau type model with a realistic geometry

In this paper, we revisit some quantum mechanical aspects related to the quantum Hall effect. We consider a Landau type model, paying a special attention to the experimental and geometrical features of quantum Hall experiments. The resulting formalism is then used to compute explicitely the Hall conductivity from a Kubo formula.     

New bounds and estimates for porous media with rigid perfectly plastic matrixNouvelles bornes et estimations pour les milieux poreux à matrice rigide parfaitement plastique

We derive new rigorous bounds and self-consistent estimates for the effective yield surface of porous media with a rigid perfectly plastic matrix and a microstructure similar to Hashin's composite spheres assemblage. These results arise from a homogenisation technique that combines a pattern-based modelling for linear composite materials and a variational formulation for nonlinear media. To cite this article: N. Bilger et al., C. R. Mecanique 330 (2002) 127–132.     

LGS and GaPO4 piezoelectric crystals: New results

Theoretical and experimental results of two interesting piezoelectric crystals for vibrating sensors, the La₅Ga₃SiO₁₄ (also written LGS) and the GaPO₄, are presented. First a comparison of different piezoelectric crystals in terms of quality factor and temperature stability is given and shows the theoretical interest of the GaPO₄ and the LGS isomorphs (LGX). Then an experimental validation has been undertaken: a dedicated decoupling structure allowing high insulation of two flexural modes is described, as well as its micromachining by ultrasonic machining (USM). The experimental measurements of the LGS resonator gave disappointing results with very low measured quality factors of about 10,000 (under vacuum) for a beam working at 10 kHz. On the other hand, the GaPO₄ resonator behaves very well: a quality factor higher than 700,000 (beam frequency 9 kHz) at room temperature and under vacuum has been obtained, as well as a low temperature frequency dependence of ?12 ppm/K, in accordance with the theoretical predictions. The impact of the Au excitation/detection electrodes on the quality factor has also been studied in detail and it clearly explains why the measured quality factors are lower than the thermoelastic limit.     

Correlation-based imaging technique using ultrasonic transmit–receive array for Non-Destructive Evaluation

This paper describes a novel array post-processing method for Non-Destructive Evaluation (NDE) using phased-array ultrasonic probes. The approach uses the capture and processing of the full matrix of all transmit–receive time-domain signals from a transducer array as in the case of the Total Focusing Method (TFM), referred as the standard of imaging algorithms. The proposed technique is based on correlation of measured signals with theoretical propagated signals computed over a given grid of points. In that case, real-time imaging can be simply implemented using discrete signal product. The advantage of the present technique is to take into account transducer directivity, dynamics and complex propagation patterns, such that the number of required array elements for a given imaging performance can be greatly reduced. Numerical and experimental application to contact inspection of isotropic structure is presented and real-time implementation issues are discussed.