Automated,Ultra‐Fast Laser‐Drilling of Nanometer Scale Pores and Nanopore Arrays in Aqueous Solutions

The ability to quickly and reliably fabricate nanoscale pore arrays in ultra‐thin membranes such as silicon nitride (Si_xN) is extremely important for the growing field of nanopore biosensing. Laser‐based etching of thin Si_xN membranes immersed in aqueous solutions has recently been demonstrated as a method to produce stable functional pores. Herein, the principal mechanism governing material etching and pore formation using light is investigated. It is found that the process is extremely sensitive to the relative content of Si over N atoms in the amorphous membrane, produced by chemical vapor deposition. Commonly, Si_xN membranes are made to be Si‐rich to increase their mechanical stability, which substantially reduces the material's bandgap and increases the density of Si‐dangling bonds. Hence, even minimal batch‐to‐batch variation may lead to remarkably different etch rates. It is shown that higher Si content results in orders of magnitude faster etching rates. This rate is further accelerated in an alkaline environment allowing on‐demand controlled nanopore formation in about 10 s time even at low laser radiation intensities. These results highlight that photoactivation of the Si_xN by the incident beam is critical to the chemical etching process and can be used to readily produce nanopore arrays at any specific location.     

Thermodynamic theory of dislocation-mediated plasticity

We reformulate the theory of polycrystalline plasticity, in externally driven, nonequilibrium situations, by writing equations of motion for the flow of energy and entropy associated with dislocations. Within this general framework, and using a minimal model of thermally assisted depinning with essentially only one adjustable parameter, we find that our theory fits the strain-hardening data for Cu over a wide range of temperatures and six decades of strain rate. We predict the transition between stage II and stage III hardening, including the observation that this transition occurs at smaller strains for higher temperatures. We also explain why strain-rate hardening is very weak up to large rates; and, with just one additional number, we accurately predict the crossover to power-law rate hardening in the strong-shock regime. Our analysis differs in several important respects from conventional dislocation-mediated continuum theories. We provide some historical background and discuss our rationale for these differences.     

Carbonization of PAN grafted uniform crosslinked polystyrene microspheres

Micrometer-sized polystyrene/poly(styrene-divinylbenzene) and polystyrene/polydivinylbenzene composite particles of narrow size distribution were formed by a single-step swelling process of uniform polystyrene template particles with emulsion droplets of dibutyl phthalate containing benzoyl peroxide and divinylbenzene in the presence or absence of styrene, followed by polymerization of the monomer(s) within the swollen template particles at 70 °C. Porous poly(styrene-divinylbenzene) and polydivinylbenzene uniform microspheres were formed by dissolution of the polystyrene part of the former composite particles. Hydroperoxide conjugated microspheres were formed by ozonolysis of the former porous microspheres. Uniform poly(styrene-divinylbenzene)/PAN and polydivinylbenzene/PAN core/shell microspheres were prepared by room temperature redox graft polymerization of AN onto the hydroperoxide conjugated particles. Uniform carbon microspheres were prepared by carbonization of the core/shell particles at 800 and 1100 °C under dynamic N₂ atmosphere. On the other hand, a similar treatment of the core particles only resulted in destruction of the particle shape. Carbon microspheres of increasing surface area (up to ca. 1000 m²/g) were prepared by activation of the former carbon microspheres with CO₂ at 850 °C. The influence of the carbonization temperature of the core/shell particles and the activation time of the carbon particles on the carbon yield and surface area has been elucidated.     

A Stepwise Open-Ended Approach to the Identification of Joint Management Structures for Shared Aquifers

The need for joint management of increasingly stressed cross-boundary aquifers is likely to grow in upcoming years. Yet there is a lack of experience in the management of cross-boundary groundwater. Moreover, the differences in the nature of groundwater and surface water preclude direct transfer of experience gained in managing cross-boundary rivers and lakes to the management of cross-border groundwater basins. This paper suggests an open-ended approach to the identification of joint management structures for shared aquifers to overcome the inability to rely on prior experience. A flexible-sequential framework is proposed that allows the parties to identify and evaluate options not tried before. The discussion is on the most salient points for analysts and decision-makers. The framework is developed within the Israeli-Palestinian context; however, the approach is applicable to a wider array of situations.     

DSP应用程序中C代码和汇编代码的结合

随着DSP处理器的功能日益强大,加上编译器的优化技术不断提高,以往只利用汇编语言编写DSP应用程序的普遍做法已逐渐被淘汰.今天,几乎所有的DSP应用程序都是由C代码和汇编代码共同构成.     

Critical Practice: Protocol for a Fused Technology

Software created for architecture often presumes a certain repertoire of design intents; software appropriated from other fields does not make such presumptions but introduces translation difficulties into material logics. Aaron Sprecher, Chandler Ahrens and Eran Neuman describe how their collaborative design group, Open Source Architecture (O-S-A), has worked with structural engineers and software developers to create a computational protocol based on a stochastic evolutionary topological optimisation algorithm. This procedure allows architectural designers to work with complex geometries previously limited to the expertise of engineering. In addition to taking the form of a distributed network spanning New York, Los Angeles and Tel Aviv, O-S-A opens the possibility for various disciplines (computer science, structural engineering, architectural design) to work with a fluidity and commonality not previously possible. Like the work of RSG (also featured in this issue), here design begins with the construction of a software environment in which a project can occur, and which merges previously distinct areas of expertise. Copyright © 2006 John Wiley & Sons, Ltd.     

Direct observation of the temporal and spatial dynamics during crumpling

Crumpling occurs when a thin deformable sheet is crushed under an external load or grows within a confining geometry. Crumpled sheets have large resistance to compression and their elastic energy is focused into a complex network of localized structures. Different aspects of crumpling have been studied theoretically, experimentally and numerically. However, very little is known about the dynamic evolution of three-dimensional spatial configurations of crumpling sheets. Here we present direct measurements of the configurations of a fully elastic sheet evolving during the dynamic process of crumpling under isotropic confinement. We observe the formation of a network of ridges and vertices into which the energy is localized. The network is dynamic. Its evolution involves movements of ridges and vertices. Although the characteristics of ridges agree with theoretical predictions, the measured accumulation of elastic energy within the entire sheet is considerably slower than predicted. This could be a result of the observed network rearrangement during crumpling.     

Apportioned margin approach for cost sensitive large margin classifiers

We consider the problem of cost sensitive multiclass classification, where we would like to increase the sensitivity of an important class at the expense of a less important one. We adopt an apportioned margin framework to address this problem, which enables an efficient margin shift between classes that share the same boundary. The decision boundary between all pairs of classes divides the margin between them in accordance with a given prioritization vector, which yields a tighter error bound for the important classes while also reducing the overall out-of-sample error. In addition to demonstrating an efficient implementation of our framework, we derive generalization bounds, demonstrate Fisher consistency, adapt the framework to Mercer’s kernel and to neural networks, and report promising empirical results on all accounts.

     

Combined top-down/bottom-up segmentation

We construct a segmentation scheme that combines top-down with bottom-up processing. In the proposed scheme, segmentation and recognition are intertwined rather than proceeding in a serial manner. The top-down part applies stored knowledge about object shapes acquired through learning, whereas the bottom-up part creates a hierarchy of segmented regions based on uniformity criteria. Beginning with unsegmented training examples of class and non-class images, the algorithm constructs a bank of class-specific fragments and determines their figure-ground segmentation. This bank is then used to segment novel images in a top-down manner: the fragments are first used to recognize images containing class objects, and then to create a complete cover that best approximates these objects. The resulting segmentation is then integrated with bottom-up multi-scale grouping to better delineate the object boundaries. Our experiments, applied to a large set of four classes (horses, pedestrians, cars, faces), demonstrate segmentation results that surpass those achieved by previous top-down or bottom-up schemes. The main novel aspects of this work are the fragment learning phase, which efficiently learns the figure-ground labeling of segmentation fragments, even in training sets with high object and background variability; combining the top-down segmentation with bottom-up criteria to draw on their relative merits; and the use of segmentation to improve recognition.