A catalyst-free new polyol method synthesized hot-pressed Cu-doped Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanorods and their thermoelectric properties

This is the first report on the thermoelectric properties of a catalyst-free polyol method used to prepare stoichiometric Bi_2?x Cu _x S₃ samples, x = 0.1, 0.2, 0.3, 0.4, via hot pressing. Various systematic approaches to arrive at in their stoichiometric compositions are explored precisely with introduction of excess precursor of S. X-ray diffraction data analysis using Rietveld refinement confirms a polyhedral orthorhombic crystal structure with a space group Pnma, in contrast to Pbnm reported earlier. Raman data further substantiates this. X-ray photoelectron spectroscopy confirms the valence states of the constituent elements (Bi³⁺, Cu²⁺, and S^2?) and energy dispersive X-ray analysis corroborates their compositions. The particle sizes of the pure Bi₂S₃ nanoparticles were 20, 35, and 82 nm as determined from the Scherrer formula, atomic force microscopy, and dynamic light scattering, respectively. Their transmission electron microscopy image shows rod-like nanostructures elongated in the 〈010〉 direction with an average diameter of 23 nm and a length of several hundreds of nanometers. A 34% improvement in the thermoelectric figure of merit is observed for Bi_1.6Cu_0.4S₃ as compared to pure Bi₂S₃ at 300 K.

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Effect of impact energy on the shape of granular heaps

We study experimentally the shape of a granular heap formed pouring a granular material into a vertical Hele-Shaw cell and analyze the effect of the grain impact energy. We propose a continuous model for the steady profile of the heap that explicitly considers energy dissipation of flowing grains through inelastic collisions. We solve the model analytically and analyze the resulting height profile as a function of several parameters, such as the restitution coefficient of the grains and their impact energies. We find good agreement between theory and experiments. Received: 9 June 1999     

Aligning scan acquisition circles in optical coherence tomography images of the retinal nerve fibre layer

Optical coherence tomography (OCT) is widely used in the assessment of retinal nerve fibre layer thickness (RNFLT) in glaucoma. Images are typically acquired with a circular scan around the optic nerve head. Accurate registration of OCT scans is essential for measurement reproducibility and longitudinal examination. This study developed and evaluated a special image registration algorithm to align the location of the OCT scan circles to the vessel features in the retina using probabilistic modelling that was optimised by an expectation-maximization algorithm. Evaluation of the method on 18 patients undergoing large number of scans indicated improved data acquisition and better reproducibility of measured RNFLT when scanning circles were closely matched. The proposed method enables clinicians to consider the RNFLT measurement and its scan circle location on the retina in tandem, reducing RNFLT measurement variability and assisting detection of real change of RNFLT in the longitudinal assessment of glaucoma.     

Improved Thermoelectric Properties of Se-Doped n-Type PbTe1−x Se x (0 ≤ x ≤ 1)

Enhancement of the thermoelectric figure of merit is of prime importance for any thermoelectric material. Lead telluride has received attention as a potential thermoelectric material. In this work, the effect of Se substitution has been systematically investigated in PbTe_1?x Se _x . The thermoelectric properties of synthesized alloys were measured in the temperature range of 300 K to 873 K. For the particular composition of x = 0.5, α was highest at ~292 μV/K, while k was lowest at ~0.75 W/m-K, resulting in the highest dimensionless figure of merit of ZT ≈ 0.95 at 600 K. The increase in thermopower for x = 0.5 can be attributed to the high distortion in the crystal lattice which leads to the formation of defect states. These defect states scatter the majority charge carriers, leading to high thermopower and high electrical resistivity. The dramatic reduction of the thermal conductivity for x = 0.5 can be attributed to phonon scattering by defect states.     

Fast timing-based algorithms

Summary.  Concurrent systems in which there is a known upper bound Δ on memory access time are considered. Two prototypical synchronization problems, mutual exclusion and consensus, are studied, and solutions that have constant (i.e. independent of Δ and the total number of processes) time complexity in the absence of contention are presented. For mutual exclusion, in the absence of contention, a process needs only five accesses to the shared memory to enter its critical section, and in the presence of contention, the winning process may need to delay itself for 4 ⋅ Δ time units. For consensus, in absence of contention, a process decides after four accesses to the shared memory, and in the presence of contention, it may need to delay itself for Δ time units. Received: July 1993/Accepted: February 1996     

Conference report. Bioengineering in the millennium. National Institute of Health symposium bioengineering--building the future of biology and medicine

Reducing the Risk is a theory-based, sexuality education curriculum shown to influence the knowledge and behaviors of secondary students. This study determined whether the behavioral effects of the curriculum could be duplicated in a southern, rural state. In a quasiexperimental design, pretest and posttest inventories were administered to students in treatment and comparison groups to determine the influence of Reducing the Risk on sexual behaviors. Results of the 18-month study indicated students receiving the curriculum significantly delayed initiating sexual intercourse. Sexually active students in the treatment group were significantly more likely to protect themselves from STD/HIV and pregnancy than sexually active students in the comparison group. In addition, students receiving Reducing the Risk showed a significant increase in parent-child communication about sexual issues. These results reinforce previous research that found positive behavioral effects for students receiving the Reducing the Risk curriculum.     

Conservative Visibility and Strong Occlusion for Viewspace Partitioning of Densely Occluded Scenes

Computing the visibility of out-door scenes is often much harder than of in-door scenes. A typical urban scene, for example, is densely occluded, and it is effective to precompute its visibility space, since from a given point only a small fraction of the scene is visible. The difficulty is that although the majority of objects are hidden, some parts might be visible at a distance in an arbitrary location, and it is not clear how to detect them quickly. In this paper we present a method to partition the viewspace into cells containing a conservative superset of the visible objects. For a given cell the method tests the visibility of all the objects in the scene. For each object it searches for a strong occluder which guarantees that the object is not visible from any point within the cell. We show analytically that in a densely occluded scene, the vast majority of objects are strongly occluded, and the overhead of using conservative visibility (rather than visibility) is small. These results are further supported by our experimental results. We also analyze the cost of the method and discuss its effectiveness.     

Tackling calibration problems of spectroscopic analysis in high-throughput experimentation

High-throughput experimentation and screening methods are changing work flows and creating new possibilities in biochemistry, organometallic chemistry, and catalysis. However, many high-throughput systems rely on off-line chromatography methods that shift the bottleneck to the analysis stage. On-line or at-line spectroscopic analysis is an attractive alternative. It is fast, noninvasive, and nondestructive and requires no sample handling. The disadvantage is that spectroscopic calibration is time-consuming and complex. Ideally, the calibration model should give reliable predictions while keeping the number of calibration samples to a minimum. In this paper, we employ the net analyte signal approach to build a calibration model for Fourier transform near-infrared measurements, using a minimum number of calibration samples based on blank samples. This approach fits very well to high-throughput setups. With this approach, we can reduce the number of calibration samples to the number of chemical components in the system. Thus, the question is no longer how many but which type of calibration samples should one include in the model to obtain reliable predictions. Various calibration models are tested using Monte Carlo simulations, and the results are compared with experimental data for palladium-catalyzed Heck cross-coupling.     

Tight bounds for shared memory systems accessed by Byzantine processes

We provide efficient constructions and tight bounds for shared memory systems accessed by n processes, up to t of which may exhibit Byzantine failures, in a model previously explored by Malkhi et al. [21]. We show that sticky bits are universal in the Byzantine failure model for n ≥ 3t + 1, an improvement over the previous result requiring n ≥ (2t + 1)(t + 1). Our result follows from a new strong consensus construction that uses sticky bits and tolerates t Byzantine failures among n processes for any n ≥ 3t + 1, the best possible bound on n for strong consensus. We also present tight bounds on the efficiency of implementations of strong consensus objects from sticky bits and similar primitive objects. Research supported in part by a grant from the Israel Science Foundation, and by the Hermann Minkowski Minerva Center for Geometry at Tel Aviv University. This work was partially completed while at AT&T Labs and while visiting the Institute for Advanced Study, Princeton, NJ. Research supported in part by US-Israel Binational Science Foundation Grant 2002246. This work was partially completed while visiting AT&T Labs. This work was partially completed while at AT&T Labs. Research supported in part by the National Science Foundation under Grant No. CCR-0331584. A preliminary version of the results presented in this paper appeared in [23].     

Detailed Mechanistic Studies using in situ Spectroscopic Analysis: A Look at Little‐Known Regions of the Heck Reaction

The mechanism of the Heck reaction using palladium complexes with large phosphoramidite ligands is investigated. The catalyst precursor is an inactive dimer that equilibrates with the active monomeric species. A series of kinetic models is introduced and compared with concentration profiles obtained from FT‐NIR spectroscopy. First, an analytical solution of the differential equations for a simplified mechanism is considered. This fits well at low conversions but deviates at higher conversions with increasing deactivation. Formation of palladium clusters and palladium black is then included, with the simplification that all the deactivation processes are represented by a single first‐order process. This results in a five‐step mechanism that describes the dimer‐monomer equilibrium, the Pd(II)/Pd(0) catalytic cycle, and the catalyst deactivation process. The model can be used to monitor the transient concentrations of the virtual Pd(0), Pd(II), and dimer catalyst species, and helps to explain the effects of water. The high resolution of the measurements and low error levels of the models render this approach a powerful tool for mechanistic studies in homogeneous catalysis.