Connectivity and percolation behaviour of grain boundary networks in three dimensions

Grain boundary networks are subject to crystallographic constraints at both triple junctions (first-order constraints) and quadruple nodes (second-order constraints). First-order constraints are known to influence the connectivity and percolation behaviour in two-dimensional grain boundary networks, and here we extend these considerations to fully three-dimensional microstructures. Defining a quadruple node distribution (QND) to quantify both the composition and isomerism of quadruple nodes, we explore how the QNDs for crystallographically consistent networks differ from that expected in a randomly assembled network. Configurational entropy is used to quantify the relative strength of each type of constraint (i.e., first- and second-order), with first-order triple junction constraints accounting for at least 75% of the non-random correlations in the network. As the dominant effects of constraint are captured by considering the triple junctions alone, a new analytical model is presented which allows the 3-D network connectivity to be estimated from data on 2-D microstructural sections. Finally, we show that the percolation thresholds for 3-D crystallographically consistent networks differ by as much as ±0.07 from those of standard percolation theory.     

Cytotoxicity of Essential Oil Cordia verbenaceae against Leishmania brasiliensis and Trypanosoma cruzi

The species Cordia verbenacea DC (Boraginaceae), known as the whaling herb and camaradinha, is a perennial shrub species native to the Atlantic Forest. Its leaves are used in folk medicine as an anti-inflammatory, analgesic, antiulcerogenic and curative agent, in the form of teas or infusions for internal or topical use. The present study aimed to verify the cytotoxicity of the essential oil and the leishmanicidal and trypanocidal potential of C. verbenacea. The essential oil was characterized by GC-MS. The in vitro biological activity was determined by anti-Leishmania and anti-Trypanosoma assays. The cytotoxixity was determined using mammalian fibroblasts. The C. verbenacea species presented α-pinene (45.71%), β-caryophyllene (18.77%), tricyclo[2,2,1-(2.6)]heptane (12.56%) as their main compounds. The essential oil exhibited strong cytotoxicity at concentrations below 250 μg/mL (LC₅₀ 138.1 μg/mL) in mammalian fibroblasts. The potent anti-trypanosome and anti-promastigote activities occurred from the concentration of 62.5 μg/mL and was considered clinically relevant. The results also demonstrate that at low concentrations (<62.5 μg/mL), the essential oil of C. verbenacea managed to be lethal for these activities. This can be considered an indication of the power used in daily human consumption. Therefore, it can be concluded that the essential oil of C. verbenacea contains a compound with remarkable antiparasitic activities and requires further research.     

Connectivity and percolation in simulated grain-boundary networks

Random percolation theory is a common basis for modelling intergranular phenomena such as cracking, corrosion or diffusion. However, crystallographic constraints in real microstructures dictate that grain boundaries are not assembled at random. In this work a Monte Carlo method is used to construct physically realistic networks composed of high-angle grain boundaries that are susceptible to intergranular attack, as well as twin-variant boundaries that are damage resistant. When crystallographic constraints are enforced, the simulated networks exhibit triple-junction distributions that agree with experiment and reveal the non-random nature of grain-boundary connectivity. The percolation threshold has been determined for several constrained boundary networks and is substantially different from the classical result of percolation theory; compared with a randomly assembled network, about 50-75% more resistant boundaries are required to break up the network of susceptible boundaries. Triple-junction distributions are also shown to capture many details of the correlated percolation problem and to provide a simple means of ranking microstructures.     

Molecular Triangles: Synthesis,Self‐Assembly,and Blue Emission of Cyclo‐7,10‐tris‐triphenylenyl Macrocycles

A set of cyclo‐7,10‐tris‐triphenylenyl macrocycles have been prepared by a Yamamoto cyclotrimerization protocol. In these novel macrocycles, three triphenylene units are covalently linked to each other, resulting in the formation of triangular‐shaped molecules. The fully planar derivative revealed pronounced self‐assembly behavior. NMR spectroscopy was used to determine the association constant in solution. 2D wide‐angle X‐ray scattering was applied to the study of the liquid crystallinity of this new discotic mesogen in the bulk state. Furthermore, nonplanar, laterally substituted derivatives were successfully tested as blue emitters in organic light‐emitting diodes owing to their unique optoelectronic properties and their high stability. In this case, substitution with sterically demanding phenyl groups was efficiently used to suppress intermolecular packing, thus preventing undesired quenching effects.     

Nonlocal superelastic model of size-dependent hardening and dissipation in single crystal Cu-Al-Ni shape memory alloys

We propose a nonlocal continuum model to describe the size-dependent superelastic effect observed in recent experiments of single crystal Cu-Al-Ni shape memory alloys. The model introduces two length scales, one in the free energy and one in the dissipation, which account for the size-dependent hardening and dissipation in the loading and unloading response of micro- and nanopillars subject to compression tests. The information provided by the model suggests that the size dependence observed in the dissipation is likely to be associated with a nonuniform evolution of the distribution of the austenitic and martensitic phases during the loading cycle.     

Controlled positioning of self-assembled InAs quantum dots on (1 1 0) GaAs

We report on a new approach for positioning of self-assembled InAs quantum dots on (1 1 0) GaAs with nanometer precision. By combining self-assembly of quantum dots with molecular beam epitaxy on in situ cleaved surfaces (cleaved-edge overgrowth) we have successfully fabricated arrays of long-range ordered InAs quantum dots. Both atomic force microscopy and micro-photoluminescence measurements demonstrate the ability to control position and ordering of the quantum dots with epitaxial precision as well as size and size homogeneity. Furthermore, photoluminescence investigations on dot ensembles and on single dots confirm the high homogeneity and the excellent optical quality of the quantum dots fabricated.     

Heterostructures overgrown on GaAs corner substrates

By choosing suitable crystal facets we are able to epitaxially overgrow a precleaved corner-substrate. We are using GaAs (1 1 0)-like facets and growth conditions such that no accumulation or depletion of deposited material near the corner is observed, avoiding morphological changes at the corner during the growth process. So we achieve high-quality layer growth across the corner. With this technique we demonstrate a new type of quantum confinement structure consisting of a GaAs/AlGaAs heterostructure overgrown on top of this precleaved corner-substrate.     

Double-resonance Semiconductor Superlattice Parametric Oscillator for Generation of Subterahertz Radiation

We report on a subterahertz superlattice parametric oscillator that operated simultaneously at two different harmonic frequencies of a microwave pump field. A pump field (frequency near 100 GHz) was coupled to a GaAs/AlAs superlattice in a resonator for the third and the fifth harmonic. The pump field produced a third harmonic field and this together with the pump field created a fifth harmonic field. A theoretical analysis indicates that the nonlinearity, which is based on the dynamics of miniband electrons, should allow for the upconversion of pump radiation of higher frequency into the terahertz frequency range.