No abstract is provided for this article.
The iridium complex Cp*(PMe3)Ir(Me)(SiMe2OTf) (2) reacts with LiB(C6F5)4(Et2O)2 to form a mixture of the silene complex [Cp*(PMe3)Ir(η2-CH2SiMe2)(H)][B(C6F5)4] (3) and the base-stabilized silylene complex [Cp*(PMe3)Ir(Me)(SiMe2(Et2O))][B(C6F5)4] (4). The silene complex 3 has been crystallographically characterized. Addition of pyridine to this mixture affords Cp*(PMe3)Ir(Me)(SiMe2(L)][B(C6F5)4] (5a, L = pyridine) in good yield, whereas addition of CO or ethylene results in clean formation of Cp*(PMe3)Ir(L)(SiMe3)][B(C6F5)4] (8a, L = CO; 8b, L = C2H4).
The objective of the AFaSR-MURI High-Cycle Fatigue program is to characterize and model the limiting damage states at the onset of high-cycle fatigue to facilitate mechanistic understanding and to develop a basis for life prediction. Efforts have been focused on the influence of foreign object damage (FAD) and fretting on a Ti-6A1-4V blade alloy and on a polycrystalline Ni-base disk alloy. Notable highlights during the fourth year include the characterization and quantitative modeling of fretting and FaD and the definition of the role of mixed-mode loading on HCF thresholds in Ti-6A1-4V. Accomplishments are outlined below: Worst-case fatigue threshold stress intensities have been measured in STOA Ti-6A1-4V using large (> 5 mm) cracks under representative HCF conditions (R > 0.95, 1000 Hz). Values provide a practical, frequency-independent (20 - 20,000 Hz) lower-bound for the growth of naturally-initiated, physically-small (> 40 gm) cracks.
Entrevista realizada a Loïc Wacquant, publicada en <i>KX-Magazine</i> (Sofía, Bulgaria, 2001), cuyo texto fue gentilmente cedido por el autor a la revista.
The diagnosis of acute stress disorder (ASD) was introduced to describe initial trauma reactions that predict chronic posttraumatic stress disorder (PTSD). This review outlines and critiques the rationales underpinning the ASD diagnosis and highlights conceptual and empirical problems inherent in this diagnosis. The authors conclude that there is little justification for the ASD diagnosis in its present form. The evidence for and against the current emphasis on peritraumatic dissociation is discussed, and the range of biological and cognitive mechanisms that potentially mediate acute trauma response are reviewed. The available evidence indicates that alternative means of conceptualizing acute trauma reactions and identifying acutely traumatized people who are at risk of developing PTSD need to be considered.
Summary General characterization of physical systems uses two aspects of data analysis methods: decomposition of empirical data to determine system parameters and reconstruction of the system attributes using these characteristic parameters. Spectral methods, involving a frequency-based representation of data, usually assume stationarity. These methods, therefore, extract only average information and, hence, are not suitable for analyzing data with isolated or deterministic discontinuities, such as faults or fractures in reservoir rocks or image edges in computer vision. Wavelet transforms provide a multiresolution framework for data representation. They are a family of basis functions that separate a function or a signal into distinct frequency packets that are localized in the time domain. Thus, wavelets are well suited for analyzing nonstationary data. In other words, a function or a discrete data set when transformed into a time-scale space using wavelets shows how it behaves at different scales of measurement. Because wavelets have compact support, it is easy to apply this transform to large data sets with minimal computations. We apply wavelet transforms to one-dimensional and twodimensional permeability data to determine the locations of layer boundaries and other discontinuities. By binning in the time-frequency plane with wavelet packets, permeability structures of arbitrary size are analyzed. Wavelets are also applied to scaling up spatially correlated heterogeneous permeability fields.
A study has been made of the effect of single compression cycles on near-threshold fatigue crack propagation in an I/M 7150 aluminum alloy. Based on experiments at a load ratio of R= 0.10 on cracks arrested at the fatigue threshold (δk th ) in under-, peak and overaged microstructures, large compression overload cycles, of magnitude five times the peak tensile load, were found to cause immediate reinitiation of crack growth, even though the applied stress intensity range did not exceed ΔK th . Following an initial acceleration, subsequent crack advance was observed to take place at progressively decreasing growth rates until rearrest occurred. Such behavior is attributed to measured changes in crack closure which vary the effective near-tip driving force for crack extension (ΔK eff ). Specifically, roughness-induced closure primarily is reduced by the application of compressive cycles via a mechanism involving crack surface abrasion which causes flattening and cracking of fracture surface asperities. Closure, however, is regenerated on subsequent propagation resulting in the rearrest. Such observations provide further confirmation that the existence of a fatigue threshold is controlled principally by the development of crack closure and are discussed in terms of the mechanisms of closure in precipitation hardened alloys.
The most important observed characteristics are described of active galactic nuclei, concentrating on their optical spectra. Connections with the standard model of such objects are emphasized. Some of the new observational developments that are leading to modifications of the standard model are outlined; these include spatial variations of the ionization parameters in the broad line region, the possible existence of a very dense component in the broad line region, and the presence of an intermediate density zone between the broad line and narrow line regions. Evidence for hidden Seyfert 1 nuclei and anisotropic ionizing radiation is also reviewed. The search for, and properties of, intrinsically weak Seyfert 1 nuclei are summarized, with emphasis given to the very low luminosity object in the late type dwarf galaxy NGC 4395. A discussion is also presented of the possibility that low level activity in some galactic nuclei might actually be produced by bursts of star formation and their associated supernovae.
There is a fundamental conductance ÷ voltage limit in low voltage (<;4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T/q) tunneling switches that turn off by relying upon the band edges to cutoff the available density of states. The Fermi occupation probabilities are thermally broadened by 4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T. However, current is only allowed to flow in a narrow energy range limited by the applied voltage, V. This means that if a voltage less than 4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T/q is applied, the conductance will be reduced by at least qV/4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T. Even with a perfect tunneling probability of 1 in a perfect quantum channel, the conductance quantum would be diminished by qV/4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T. Attempts at lowering the operating voltage below <;4k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> T/q must come at the expense of smaller conductance.
