A cluster approach to hydrogen chemisorption on the GaAs(1 0 0) surface

Chemisorption properties of atomic hydrogen on the Ga-rich GaAs(1 0 0), (2×1) and β(4×2) surfaces are investigated using ab initio self-consistent restricted open shell Hartree–Fock (ROHF) total energy calculations with Hay–Wadt (HW) effective core potentials. The effects of electron correlation have been included using many-body perturbation theory through second order with the exception of β(4×2) symmetry due to computational limitations. The semiconductor surface is modeled by finite sized hydrogen saturated clusters. The effects of surface reconstruction have been investigated in detail. We report on the energetics of chemisorption on the (1 0 0) surface layer, including adsorption beneath the surface layer at an interstitial site, and also report on the possible dimer bond breaking at the bridge site. Chemisorption energies, bond lengths, and charge population analysis are reported for all considered sites of chemisorption.     

Bainite morphologies in a 0.2 C–1.5 Mn steel

The isothermal transformation products of austenite over a wide range of temperatures and times in the bainitic range in a 0.2 wt.% C–1.5 wt.% Mn steel have been studied by transmission electron microscopy in order to characterise the bainitic microstructures in low-carbon low-alloy steels. Widmanstätten ferrite has formed with alternate layers of austenite (martensite) as a transition product at 600 and 500°C that has finally transformed on further isothermal transformation to either pearlite (at 600°C) or upper bainite (at 500°C). This type of transformation product was referred to as BI bainite by earlier investigators, but on the basis of the present investigation it is concluded that such ferrite-austenite (martensite) structures are not bainitic as this is not the final transformation product either at 600 or 500°C. Both upper bainite and lath-type lower bainite are formed at 450°C while the transformation product has been only lath-type lower bainite at 400°C.     

Performance management of manufacturing system networks

Performance management of communication networks is critical for speed, reliability, and flexibility of information exchange between different components, subsystems, and sectors (e.g., factory, engineering design, and administration) of production process organizations in the environment of computer integrated manufacturing (CIM). Essential to this distributed total manufacturing system is the integrated communications network over which the information leading to process interactions and plant management and control is exchanged. Such a network must be capable of handling heterogeneous traffic resulting from intermachine communications at the factory floor, CAD drawings, design specifications, and administrative information. The objective is to improve the efficiency in handling various types of messages, e.g., control signals, sensor data, and production orders, by on-line adjustment of the parameters of the network protocol.This paper presents a conceptual design, development, and implementation of a network performance management scheme for CIM applications including flexible manufacturing. The performance management algorithm is formulated using the concepts of: (1) Perturbation analysis of discrete event dynamic systems; (2) stochastic approximation; and (3) learning automata. The proposed concept for performance management can also serve as a general framework to assist design, operation, and management of flexible manufacturing systems.The performance management procedure has been tested via emulation on a network test bed that is based on the manufacturing automation protocol (MAP) which has been widely used for CIM networking. The conceptual design presented in this paper offers a step forward to bridging the gap between management standards and users' demands for efficient network operations since most standards such as ISO and IEEE address only the architecture, services, and interfaces for network management.This work was supported in part by National Science Foundation under Research Grant No. DDM-90-15173.     

A State-Space Model of Fatigue Crack Growth

This paper proposes a nonlinear dynamic model of fatigue crack growth in the state-space setting based on the crack closure concept under cyclic stress excitation of variable amplitude and random loading. The model state variables are the crack length and the crack opening stress. The state-space model is capable of capturing the effects of stress overload and underload on crack retardation and acceleration, and the model predictions are in fair agreement with experimental data on the 7075-T6 aluminum alloy. Furthermore, the state-space model recursively computes the crack opening stress via a simple functional relationship and does not require a stacked array of peaks and valleys of stress history for its execution; therefore, savings in both computation time and memory requirement are significant. As such, the state space model is suitable for real-time damage monitoring and control in operating machinery. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new characterization technique for trimethylsilylated products of old cement pastes

Twenty-year-old and laboratory-prepared completely hydrated cement pastes were investigated by the trimethylsilylation technique. The resulting derivatives were separated by both gas and gel permeation chromatography (GPC). Separation of the non-volatile fraction of the trimethylsilyl derivatives was more efficient by the GPC technique. The trimethylsilyl derivatives of SiO₄^4? and Si₂O₇^6? ions were easily identified by GPC. The constitutions of the other silicate anion derivatives separated by GPC were inferred on the basis of their molecular weights, relative to those of SiO₄^4? and Si₂O₇^6? ions.     

An application of wavelet analysis to paleoproductivity records from the Southern Ocean

We have performed wavelet analysis of the paleoproductivity records from the southeastern Atlantic sector of the Southern Ocean. In particular, we have applied a continuous wavelet transform to the phosphorus-to-titanium (P/Ti) ratios obtained from the Ocean Drilling Program (ODP) Sites 1089 and 1094 over the past ～600 kyr, and computed their wavelet power spectra in order to elucidate the spectral–temporal aspects of phosphorus deposition at these sites. Because data are unevenly sampled in time, they are first resampled using a uniform sampling interval prior to wavelet analysis. The wavelet power spectra of the P/Ti data are presented on a time-period plane from which the dominant periodicities of P export and their duration in time are discerned by visual inspection. Our results indicate that at the ODP Site 1089, the strongest P export occurs within a periodic band with the most dominant period of ～100 kyr, which corresponds to the eccentricity period of Milankovitch cyclicity. On the other hand, at the ODP Site 1094, which is completely covered with ice during the glacial times, the strongest periodicity of P export is found to be around 82 kyr. This periodicity is twice the period of the 41-kyr Milankovitch obliquity cycle, implying that obliquity forcing drives the productivity at this site in a period doubling scenario. Given the strong influence of ice cover at this site, another implication is that sea ice extent itself is strongly influenced by the obliquity signal, and productivity simply responds to the availability of ice-free conditions. We have also performed wavelet analysis of the carbonate production data at the ODP Site 1089, and found a dominant periodicity of ～100 kyr. In addition, a few weaker decadal-scale periodicities are observed in both P/Ti and carbonate data which can be linked to Milankovitch cycles.     

PAPR reduction using twin symbol hybrid optimization-based PTS and multi-chaotic-DFT sequence-based encryption in CP-OFDM system

Photonic Network Communications - Orthogonal frequency division multiplexing (OFDM) is considered as one of the most significant transmission methodologies of the recent past. Moreover, it permits...