Grain boundary-based plasticity mechanisms in nanostructured metals

The Hall-Petch relationship establishes the proportional dependency of a metal’s strength with the inverse of its grain size’s square root. This phenomena, is well understood using dislocation-based plasticity, until the grain size becomes too small. There plastic deformation mechanisms are usually related to grain boundaries (GBs). Previous studies have observed and simulated several plastic deformation mechanisms such as grain rotation, grain sliding and shear-migration coupling (Figure 1). Some models have been proposed to predict shear-migration coupling based on the initial GB misorientations[1], [4]. However, they have not yet been proven experimentally in the case of polycrystals. This study focuses on the shear-migration coupling, a mechanism which has fueled many recent studies in the field of plasticity [1], [2], [3]. To carry it out, we use polycrystals of Aluminum, Copper and Nickel, with ultrafine grains (\u3c1µm). We aim to find experimental evidences of such mechanism and characterize it in order to correlate it with initial grain misorientations, straining rate, chemical distribution, etc. Please click Additional Files below to see the full abstract

Two caesium vanadium hydrogenphosphates with tunnelled structures: Cs2V2O3(PO4)(HPO4) and Cs2[(VO)3(HPO4)4(H2O)].H2O

International audienceDicaesium divanadium trioxide phosphate hydrogenphosphate, Cs2V2O3(PO4)(HPO4), (I), and dicaesium tris[oxidovanadate(IV)] hydrogenphosphate dihydrate, Cs2[(VO)3-(HPO4)4(H2O)] H2O, (II), crystallize in the monoclinic system with all atoms in general positions. The structures of the two compounds are built up from VO6 octahedra and PO4 tetrahedra. In (I), infinite chains of corner-sharing VO6 octahedra are connected to V2O10 dimers by phosphate and hydrogenphosphate groups, while in (II) three vanadium octahedra share vertices leading to V3O15(H2O) trimers separated by hydrogenphosphate groups. Both structures show three-dimensional frameworks with tunnels in which Cs+ cations are located

Measurement of particle and bubble accelerations in turbulence

4 pagesWe use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time resolution, 30 microseconds, resolves the fastest scales of the fluid motion. Particles are tracked for mean durations of the order of 10 Kolmogorov time scales. The fastest scales of the particle motion are resolved and the particle acceleration is measured. For neutrally buoyant particles, our measurement matches the performance of the silicon strip detector technique introduced at Cornell University~\cite{Voth,MordantCornell}. This reference dynamics is then compared to that of slightly heavier solid particles (density 1.4) and to air bubbles. We observe that the acceleration variance strongly depends on the particle density: bubbles experience higher accelerations than fluid particles, while heavier particles have lower accelerations. We find that the probability distribution functions of accelerations normalized to the variance are very close although the air bubbles have a much faster dynamics

Shear-coupling migration of grain boundaries in UFG Al

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An unknown trees die back caused by Pseudomonas species in Switzerland

A model for tree pathogen diagnosis – Prunus domestica L. has been studied against pathogenic bacteria. An orchard of 110 trees of P. domestica showed dying back symptoms in May 2009 and nineteen of these trees were eradicated and burnt for prophylaxis. No symptoms correlated with those caused by pathogens previously observed in stone fruit die back in Europe or elsewhere (Pseudomonas syringae pv syringae van Hall, Pseudomonas syringae pv morsprunorum Lazarowtz, Phytophthora sp., Diaporthe perniciosa Marchal., phytoplasma or viruses) were not found. Interestingly, cutting the trunk in transversal sections allowed the observation of stem heart necrosis which was mostly important at the grafting point. Isolations from necrotic stem heart allowed to identify anot yet described Pseudomonas species not related to P. syringae. The method described in the paper for isolation of pathogenic bacteria and their quick an reliable identification can be also applied for detection of pathogens in forest tree plantations.Przemysław Szmi

Local backbone structure prediction of proteins.

A statistical analysis of the PDB structures has led us to define a new set of small 3D structural prototypes called Protein Blocks (PBs). This structural alphabet includes 16 PBs, each one is defined by the (phi, psi) dihedral angles of 5 consecutive residues. The amino acid distributions observed in sequence windows encompassing these PBs are used to predict by a Bayesian approach the local 3D structure of proteins from the sole knowledge of their sequences. LocPred is a software which allows the users to submit a protein sequence and performs a prediction in terms of PBs. The prediction results are given both textually and graphically

Large-scale fluctuations and dynamics of the Bullard-von Karman dynamo

International audienceA synthetic fluid dynamo built in the spirit of the Bullard device (Bullard, The stability of a homopolar dynamo, Proc. Camb. Phil. Soc. 1955, 51, 744) is investigated. It is a two-step dynamo in which one process stems from the fluid turbulence, while the other part is achieved by a linear amplification of currents in external coils (Bourgoin et al., A bullard-von Karman dynamo, New J. Phys., 2006, 8, 329). Modifications in the forcing are introduced in order to change the dynamics of the flow, and hence the dynamo behavior. Some features, such as on-off intermittency at onset of dynamo action, are very robust. Large-scale fluctuations have a significant impact on the resulting dynamo, in particular in the observation of magnetic field reversals

Single-fed circularly polarized dielectric resonator antenna using a uniaxial anisotropic material

A Dielectric Resonator Antenna operating in circular polarization is presented. The proposed antenna is formed by a uniaxial anisotropic dielectric and a ground plane. The optimization of the permittivity tensor and antenna dimensions is performed both theoretically and numerically using an Eigen mode analysis in Ansys HFSS. A good agreement is obtained for the resonant frequencies and quality factors of the different modes. The uniaxial anisotropic DRA radiates two degenerate orthogonal modes, TE111x and TE111y, with equal amplitudes and 90° phase difference in order to achieve circular polarization. An impedance bandwidth of 9.8% is accomplished and a broadside radiation pattern with left-hand circular polarization (LHCP) is achieved in simulation. Furthermore, an Axial Ratio (AR) of 0.06 dB at 2.45 GHz and a 3-dB AR bandwidth of 2.04% are obtained. The maximum simulated directivity is equal to 6.56 dBi

Long-term memory in experiments and numerical simulations of hydrodynamic and magnetohydrodynamic turbulence

International audienceWe analyze time series stemming from experiments and direct numerical simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are done in periodic boxes, but with a volumetric forcing chosen to mimic the geometry of the flow in the experiments, the von Kármán swirling flow between two counterrotating impellers. Parameters in the simulations are chosen to (within computational limitations) allow comparisons between the experiments and the numerical results. Conducting fluids are considered in all cases. Two different configurations are considered: a case with a weak externally imposed magnetic field and a case with self-sustained magnetic fields. Evidence of long-term memory and 1/f noise is observed in experiments and simulations, in the case with weak magnetic field associated with the hydrodynamic behavior of the shear layer in the von Kármán flow, and in the dynamo case associated with slow magnetohydrodynamic behavior of the large-scale magnetic field