Single-sample decohesion test: mechanics and implementation

A front-tracking finite element method is used to compute the evolution of a crack-like defect that propagates along a bi-material interface by stress driven corrosion. Depending on material properties, loading, and temperature, simulations predict five possible behaviors for the flaw: (a) The notch may blunt, so that a fatigue threshold exists for the composite; (b) The flaw may branch out of the interface, and thereafter propagate as a stable notch; (c) The notch may branch out of the interface, and then progressively sharpen at its tip, with both the notch tip curvature and stress approaching unbounded values; (d) The flaw may propagate as a stable notch parallel to the interface; (e) The notch may propagate parallel to the interface, but with the tip curvature and stress progressively increasing without limit. The range of material parameters and loading conditions that leads to each type of behavior is calculated. For conditions where steady-state interfacial notch growth occurs, the tip velocity is computed as function of material and loading.     

Influence of spray drying and dispersing agent on surface and dissolution properties of griseofulvin micro and nanocrystals

The purpose for the current research is to compare and evaluate physiochemical properties of spray-dried (SD) microcrystals (MCs), nanocrystals (NCs), and nanocrystals with a dispersion agent (NCm) from a poorly soluble compound. The characterization was carried out by performing size and surface analysis, interfacial tension (at particle moisture interface), and in-vitro drug dissolution rate experiments. Nanosuspensions were prepared by media milling and were spray-dried. The SD powders that were obtained were characterized morphologically using scanning electron microscopy (SEM), polarized light microscopy (PLM), and Flowchem. Solid-state characterization was performed using X-ray powder diffraction (XRPD), Fourier transfer infrared spectroscopy (FT-IR), and differential scanning calorimetry (DSC) for the identification of the crystalline nature of all the SD powders. The powders were characterized for their redispersion tendency in the water and in pH 1.2. Significant differences in redispersion were noted for both the NCs in both dissolution media. The interfacial tension for particle moisture interface was determined by applying the BET (Braunauer–Emmett–Teller) equation to the vapor sorption data. No significant reduction in the interfacial tension was observed between MCs and NCs; however, a significant reduction in the interfacial tension was observed for NCm at both 25?°C and 35?°C temperatures. The difference in interfacial tension and redispersion behavior can be attributed to a difference in the wetting tendency for all the SD powders. The dissolution studies were carried out under sink and under non-sink conditions. The non-sink dissolution approach was found suitable for quantification of the dissolution rate enhancement, and also for providing the rank order to the SD formulations.     

Quantitative determination of higher harmonic content in the soft x-ray spectra of toroidal grating monochromator using a reflection multilayer

Use of a grating monochromator causes a problem of higher harmonic contaminations in a synchrotron beamline operating in the soft x ray/vacuum ultraviolet region. Generally gratings are used to experimentally determine the higher harmonic contaminations. In this method, the relative contribution of contaminant wavelengths is measured with respect to the first harmonic wavelength (desired wavelength). The quantitative fit of grating spectra is rather complex, and therefore qualitative analysis is carried out. Analysis of multilayer reflectivity data has become rather simple with recent advances in the theoretical modeling. Therefore we propose to use a multilayer mirror and analyze its reflectivity data for quantitative determination of harmonic contamination in a soft x ray beamline. In the present study we used a Mo/Si multilayer of d=97 ? to quantify the spectral purity of 600 lines/mm toroidal grating at the reflectivity beamline of Indus-1 450 MeV synchrotron source. The measured reflectivity spectra at each wavelength is analyzed and the actual contribution of higher harmonics in the incident beam is obtained. Details of methodology and results are discussed.     

Attitude Control of Space Platform Based Tethered Satellite System

Using an order N Lagrangian formulation, the paper studies attitude control of a rigid platform supporting a flexible tether connected to a rigid satellite. The system, in an arbitrary orbit, is free to undergo three-dimensional motion in both rigid and flexible degrees of freedom. As can be expected, the governing equations of motion, in general, are highly nonlinear, nonautonomous, and coupled, and are amenable only to numerical integration. The control is achieved through time dependent offset of the tether attachment point, as determined through the Liapunov method, thus providing regulated amount of tether tension induced damping moment. Results suggest that the controller is quite successful in stabilizing the platform about its nominal equilibrium position in a few orbits, even in the presence of relatively large disturbances. Furthermore, extensive parametric study suggests that the controller is quite versatile in imparting any desired orientation to the platform. This would enable the system to undertake diverse missions aimed at communications, launch and retrieval of spacecraft, monitoring the Earth's environment, planetary and galactic observations, etc.     

Multi-agent Path Planning with Heterogenous Interactions in Tight Spaces

By starting with the assumption that motion is fundamentally a decision making problem, we use the world-line concept from Special Relativity as the inspiration for a novel multi-agent path planning method. We have identified a particular set of problems that have so far been overlooked by previous works. We present our solution for the global path planning problem for each agent and ensure smooth local collision avoidance for each pair of agents in the scene. We accomplish this by modelling the collision-free trajectories of the agents through 2D space and time as rods in 3D. We obtain smooth trajectories by solving a non-linear optimization problem with a quasi-Newton interior point solver, initializing the solver with a non-intersecting configuration from a modified Dijkstra's algorithm. This space–time formulation allows us to simulate previously ignored phenomena such as highly heterogeneous interactions in very constrained environments. It also provides a solution for scenes with unnaturally symmetric agent alignments without the need for jittering agent positions or velocities.     

Magnetic and electrical properties of Al3+-substituted MgFe2O4

The structural, magnetic and electrical properties of the Al³⁺-substituted disordered spinel system Mg(Fe_2–x Al _x )O₄ have been investigated by X-ray diffraction, magnetization, a.c. susceptibility and electrical resistivity measurements. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained through Mössbauer spectroscopy. The variation of saturation magnetization per formula unit as a function of aluminium context, x, has been satisfactorily explained on the basis of Neel's collinear spin model and the slight discrepancy between the observed and calculated n _B values can be explained in terms of a random canting model. The Néel temperatures calculated theoretically by applying molecular field theory agreed well with the experimentally determined values from thermal variation of susceptibility and electrical resistivity. An unusual metal-like thermo-electric behaviour was found for the compositions with x 0.3 which was attributed to the decrease in the Fe-Fe separation distance arising from aluminium substitution.     

Temperature effects on resistance of aligned multiwalled carbon nanotube films

Electrical transport in vertically aligned films of multiwalled carbon nanotubes has been investigated in the -150 degrees C to 300 degrees C temperature range (all the tests were conducted in air at atmospheric pressure). In all the cases, the nanotube film exhibited a semi-conducting behavior, with the film resistance decreasing with increasing temperature. Removal of amorphous carbon contamination (via plasma etching) significantly improved the nanotube film's sensitivity to temperature changes (particularly in the 20 degrees C to 200 degrees C temperature range). All the of films tested in this study showed a consistent, repeatable behavior that was independent of the nanotube film length. The temperature sensitivity of the nanotube films was also found to be independent of the heating/cooling rates and without hysteresis. Because of the excellent repeatability and stability of the results, it is conceived that miniaturized temperature sensors could be designed using such aligned multiwalled nanotube films.     

Investigation of structural and some physical properties of Cr substituted polycrystalline Eu<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Mn<Subscript>1−x</Subscript>Cr<Subscript>x</Subscript>O<Subscript>3</Subscript> (0 ≤ x ≤ 0.1) manganites

The polycrystalline samples with nominal composition Eu_0.5Sr_0.5Mn_1?x Cr _x O₃ (0 ≤ x ≤ 0.1) were prepared by the conventional solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy and electrical resistivity behavior without and with magnetic field. The structural parameters obtained by using Rietveld refinement of X-ray diffraction data showed that all samples crystallize with orthorhombic perovskite type symmetry with Pbnm space group. The scanning electron micrograph images reveal that the increase in Cr substitution hinders grain growth and grain connectivity. The temperature dependence of electrical resistivity show the semiconducting nature of these compounds and support the small polaron hoping model and variable range hopping conduction model. The calculated hopping distance and activation energy decreased as rate of Cr content increased whereas density of states at Fermi level increased. A large negative magnetoresistance is also present in the sample at the lowest temperature of measurements.     

通过改进转换器设计来降低能耗

全球正越来越关注能耗问题。降低能耗有两种方式,其中,节能需要个人/社会有意识的制定相关策略,而改进能效则是一种纯技术性的方式,对于终端用户是透明的。