Compressive properties of pristine and SiC-Te-added MgB2 powders,green compacts and spark-plasma-sintered bulks

Pristine and (SiC+Te)-added MgB₂ powders, green and spark plasma sintered (SPS) compacts were investigated from the viewpoint of quasi-static and dynamic (Split-Hopkinson Pressure Bar, SHPB) compressive mechanical properties The amount of the additive (SiC+Te) was selected to be the optimum one for maximization of the superconducting functional parameters. Pristine and added MgB₂ show very similar compressive parameters (tan δ, fracture strength, Vickers hardness, others) and fragment size in the SHPB test. However, for the bulk SPSed samples the ratio of intergranular to transgranular fracturing changes, the first one being stronger in the added sample. This is reflected in the quasi-static K_IC that is higher for the added sample. Despite this result, sintered samples are brittle and have roughly similar fragmentation behavior as for brittle engineering ceramics. In the fragmentation process, the composite nature of our samples should be considered with a special focus on MgB₂ blocks (colonies) that show the major contribution to fracturing. The Glenn-Chudnovsky model of fracturing under dynamic load provides the closest values to our experimental fragment size data.     

PHYSICS-BASED SIMULATION OF HIGH SPEED MACHINING

Computer simulation of high speed machining processes can provide a unique insight and reduce the number of design iterations required to advance and optimize the process. Predictive modeling of high speed machining of exotic materials has been hindered by the nonlinear behavior of this type of materials at extremely high strain, strain rate, and temperatures. This paper presents a physics-based modeling technology that includes the change in the material constitutive equation and the friction characterization at cutting speeds up to 400 m min^-1. The dependence of the accuracy of the predicted parameters, such as the chip formation on cutting forces, chip/tool/workpiece interface temperature, stress and strain distributions are also discussed. The fundamentals of metal cutting were utilized to understand the effect of parameter changes in regimes that are outside current empirical knowledge databases.     

Tree-Based Concurrency Control in Distributed Groupware

We present a novel algorithm, called dARB, forsolving the concurrency control problem indistributed collaborative applications. Themain issue of concurrency control is resolvingthe conflicts resulting from simultaneousactions of multiple users. The algorithmreduces the need for manual conflict resolutionby using a distributed arbitration scheme. Themain advantages of our approach are thesimplicity of use and good responsiveness, asthere are no lock mechanisms. Our algorithmrequires the applications to use a tree as theinternal data structure. This makes itapplication independent and suitable forgeneral collaborative applications. The treerequirement is reasonable since many newapplications use XML (extensible MarkupLanguage) for data representation and exchange,and parsing XML documents results in treestructures. Example applications of thealgorithm, a group text editor and acollaborative 3D virtual environment calledcWorld, are implemented and evaluated in theDISCIPLE collaboration framework. We alsointroduce awareness widgets that users avoidgenerating the conflicting events and help inmanual conflict resolution.     

Dispersion in Disks

We present three new approximation algorithms with improved constant ratios for selecting n points in n disks such that the minimum pairwise distance among the points is maximized.

1. A very simple O(nlog?n)-time algorithm with ratio 0.511 for disjoint unit disks.
2. An LP-based algorithm with ratio 0.707 for disjoint disks of arbitrary radii that uses a linear number of variables and constraints, and runs in polynomial time.
3. A hybrid algorithm with ratio either 0.4487 or 0.4674 for (not necessarily disjoint) unit disks that uses an algorithm of Cabello in combination with either the simple O(nlog?n)-time algorithm or the LP-based algorithm.

The LP algorithm can be extended for disjoint balls of arbitrary radii in ? ^d , for any (fixed) dimension d, while preserving the features of the planar algorithm. The algorithm introduces a novel technique which combines linear programming and projections for approximating Euclidean distances. The previous best approximation ratio for dispersion in disjoint disks, even when all disks have the same radius, was 1/2. Our results give a positive answer to an open question raised by Cabello, who asked whether the ratio 1/2 could be improved.     

Novel methods for rapid assessment of tool performance in milling

The paper examines the effectiveness of two innovative techniques designed to rapidly optimize a milling application. One of them relates to quantifying the relative wear of different insert grades concurrently in a single cutting test, by mounting the inserts in the same cutter, for a quick comparative performance evaluation. Experimental results that illustrate the validity and limitation of this concept, and a scheme for enhancing the reliability of the test method are presented. The other technique refers to rapid identification of the optimum feed/tooth that corresponds to maximum tool life. This entails a test wherein individual inserts in the cutter are subject to feed/tooth that are multiples of a base value, by selectively leaving appropriate number of consecutive insert pockets unoccupied. These novel techniques complement known accelerated tool life tests, and are expedient for industries that engage short production runs, in terms of selecting a suitable insert grade for an application, and determining optimal cutting conditions for the selected grade.     

Laser-Printed Plasmonic Metasurface Supporting Bound States in the Continuum Enhances and Shapes Infrared Spontaneous Emission of Coupled HgTe Quantum Dots

In order to advance the development of quantum emitter-based devices, it is essential to enhance light-matter interactions through coupling between semiconductor quantum dots with high quality factor resonators. Here, efficient tuning of the emission properties of HgTe quantum dots in the infrared spectral region is demonstrated by coupling them to a plasmonic metasurface that supports bound states in the continuum. The plasmonic metasurface, composed of an array of gold nanobumps, is fabricated using single-step direct laser printing, opening up new opportunities for creating exclusive 3D plasmonic nanostructures and advanced photonic devices in the infrared region. A 12-fold enhancement of the photoluminescence in the 900–1700 nm range is observed under optimal coupling conditions. By tuning the geometry of the plasmonic arrays, controllable shaping of the emission spectra is achieved, selectively enhancing specific wavelength ranges across the emission spectrum. The observed enhancement and shaping of the emission are attributed to the Purcell effect, as corroborated by systematic measurements of radiative lifetimes and optical simulations based on the numerical solution of Maxwell's equations. Moreover, coupling of the HgTe photoluminescence to high quality factor modes of the metasurface improves emission directivity, concentrating output within an ≈20° angle.     

Modeling the effect of node synchronization times in ultra-wideband wireless networks

Ultra-wideband wireless (UWB) can provide the physical layer for high-throughput personal area networks. When UWB is used for communication between many nodes, relatively long acquisition times are needed when dropping and re-establishing wireless links between the nodes. This paper describes the development and use of mathematical and simulation models to investigate the impact of dropping and reacquiring links between nodes on average packet delay; we also consider the performance of the alternative strategy of forwarding packets through intermediate nodes without breaking the established wireless links. The work presented here assumes that no specific MAC layer protocol, such as WiMedia UWB MAC, is in operation. The paper describes the models, explains the selection of modeling parameters used, compares the average packet delay for a network of three simple UWB nodes and for a ring of ten UWB nodes and explains the use of these results for network design engineers.     

Clustering in ordered dissimilarity data

This paper presents a new technique for clustering either object or relational data. First, the data are represented as a matrix D of dissimilarity values. D is reordered to D * using a visual assessment of cluster tendency algorithm. If the data contain clusters, they are suggested by visually apparent dark squares arrayed along the main diagonal of an image I( D *) of D *. The suggested clusters in the object set underlying the reordered relational data are found by defining an objective function that recognizes this blocky structure in the reordered data. The objective function is optimized when the boundaries in I( D *) are matched by those in an aligned partition of the objects. The objective function combines measures of contrast and edginess and is optimized by particle swarm optimization. We prove that the set of aligned partitions is exponentially smaller than the set of partitions that needs to be searched if clusters are sought in D . Six numerical examples are given to illustrate various facets of the algorithm. © 2009 Wiley Periodicals, Inc.