Black Silicon with high density and high aspect ratio nanowhiskers

Physical properties of black Silicon (b-Si) formed on Si wafers by reactive ion etching in chlorine plasma are reported in an attempt to clarify the formation mechanism and the origin of the observed optical and electrical phenomena which are promising for a variety of applications. The b-Si consisting of high density and high aspect ratio sub-micron length whiskers or pillars with tip diameters of well under 3 nm exhibits strong photoluminescence (PL) both in visible and infrared, which are interpreted in conjunction with defects, confinement effects and near band-edge emission. Structural analysis indicate that the whiskers are all crystalline and encapsulated by a thin Si oxide layer. Infrared vibrational spectrum of Si-O-Si bondings in terms of transverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder induced LO-TO optical mode coupling can be an effective tool in assessing structural quality of the b-Si. The same phonons are likely coupled to electrons in visible region PL transitions. Field emission properties of these nanoscopic features are demonstrated indicating the influence of the tip shape on the emission. Overall properties are discussed in terms of surface morphology of the nano whiskers

Differences in assigning probabilities to coastal inundation hazard estimators: event versus response approaches

This is the accepted version of the following article: Sanuy, M, Jiménez, JA, Ortego, MI, Toimil, A. Differences in assigning probabilities to coastal inundation hazard estimators: Event versus response approaches. J Flood Risk Management. 2020; 13 (Suppl. 1):e12557. https://doi.org/10.1111/jfr3.12557, which has been published in final form at https://onlinelibrary.wiley.com/doi/full/10.1111/jfr3.12557.Coastal flood risk assessment requires a reliable estimation of the frequency of inundation hazards, that is, characterising the hazard magnitude and assigning a probability of occurrence. In this work we analyse the uncertainty introduced in the assessment associated to the method to assign the probability of occurrence to coastal flood hazards. To this end we have compared the use of two general methods, the response and the event approaches. Different procedures are used to characterise coastal inundation hazards depending on the analysis scale and data availability. Thus, a range of possibilities has been analysed, from simple estimators such as run-up to modelled flood-prone areas. The analysis has been performed for all wave and water level conditions around the Spanish coast. The results show that the differences between the methods are location-dependent, and thus, determined by the exposure to wave and water level conditions. When using the event approach, the run-up or total water level (with good correlation between waves and surge) distributions reasonably approximate those of the response approach with low associated uncertainty. When the assessment aims to output overtopping discharges or inundation maps, observed differences suggest that the event approach would produce misleading conclusions in inundation-related coastal management and decision-making.Peer ReviewedPostprint (author's final draft

A mathematical model for unsteady mixed flows in closed water pipes

We present the formal derivation of a new unidirectional model for unsteady mixed flows in non uniform closed water pipe. In the case of free surface incompressible flows, the \FS-model is formally obtained, using formal asymptotic analysis, which is an extension to more classical shallow water models. In the same way, when the pipe is full, we propose the \Pres-model, which describes the evolution of a compressible inviscid flow, close to gas dynamics equations in a nozzle. In order to cope the transition between a free surface state and a pressured (i.e. compressible) state, we propose a mixed model, the \PFS-model, taking into account changes of section and slope variation

Establishing the database of inundation potential in Taiwan

Copyright © Springer 2006. The final publication is available at Springer via http://dx.doi.org/10.1007/s11069-005-4659-7The structural measure was the major solution for flood defense in Taiwan. However, the measure is always limited to the design standard and cannot prevent the damages when floods exceed certain scale. Therefore, non-structural measures for flood mitigation are the indispensable complements to structural solutions. The study introduces the establishment of inundation potential database that provides required information for the non-structural measures in Taiwan. The database was built by numerical simulations, based on different rainfall scenarios, and has been applied by the local governments of Taiwan for land use managements, flood warning systems, emergency responses, and flood insurance programs to reduce the flood damages and impacts.National Science Council, Executive Yuan, ROC

Mechanical cleaning of graphene using in situ electron microscopy

Avoiding and removing surface contamination is a crucial task when handling specimens in any scientific experiment. This is especially true for two-dimensional materials such as graphene, which are extraordinarily affected by contamination due to their large surface area. While many efforts have been made to reduce and remove contamination from such surfaces, the issue is far from resolved. Here we report on an in situ mechanical cleaning method that enables the site-specific removal of contamination from both sides of two dimensional membranes down to atomic-scale cleanliness. Further, mechanisms of re-contamination are discussed, finding surface-diffusion to be the major factor for contamination in electron microscopy. Finally the targeted, electron-beam assisted synthesis of a nanocrystalline graphene layer by supplying a precursor molecule to cleaned areas is demonstrated

A Quasi Method of Characteristics with Applications to Fluid Lines with Frequency Dependent Wall Shear and Heat Transfer

Professor Streeter has given a fine summary of the basic numerical techniques for unsteady flows, presuming that equation One exception, well known to Professor Streeter and included in several of his references, is the simpler case of laminar rather than turbulent friction for low frequency excitation. Only minor variations in the equations are necessary. A much greater departure from equation 4 Numbers in brackets designate Additional References at end of discussion. at intermediate frequencies in turbulent flow. Apparently because of a little-understood resonance of ring vortices, the step response of a tube may contain significant oscillations. Wavelengths of the complicated patterns are about 25 and 50 diameters. (Further information is forthcoming in a thesis by Margolis.) The report also discusses the details of numerical application of the quasi method of characteristics to large amplitude transients, with illustrations. Readers should know that the paper and this discussion represent a highly selected rather than comprehensive review of the important literature on numerical methods for unsteady flow calculations in channels and tubes. T. P. Propson 6 The author has conducted a thorough review of the most popular techniques currrently employed to numerically evaluate the effect of transient flows in liquid piping systems. His discussion of the relative advantages and disadvantages of both the characteristics (explicit) and centered implicit method is excellent; of particular interest to the writer were the author's comments relative to the occurrence of instabilities and inaccuracies occasionally encountered during application of the implicit techniques. Recent unpublished work by the writer has confirmed these problems. When frictional effects are very important, the writer would suggest that equations (64) It may be shown that the error introduced into the integration of the friction term by these finite-difference equations is usually about one-half of that introduced by equations (30) and (31)

Comparaison des résultats des essais d'intumescences effectués sur le modèle réduit et sur le modèle mathématique du canal Oraison-Manosque

The first part of the article discusses the difficulties encountered in the development of mathematical models for the study of steep-fronted translation waves in power canals. The assumptions behind St. Venant's equations are briefly reviewed. The non-linear nature of these equations is brought out and also the consequences which this entails, in particular, the fact that after a certain time there can be no continuity in their solution. The steepening of the wave invalidates the flow theory which ignores vertical acceleration. The use of the method of characteristics to compute the moving hydraulic jump is rejected after a discussion of the difficulties involved in its application to practical cases. Finite difference methods can however be used for this type of computation. The section of the Oraison canal represented by the scale and mathematical models contains several transitions, including sills, a lateral spillweir, a flow divider and a syphon. Although it was possible to express the roughness of the running sections of the canal in the mathematical model with the aid of known Strickler coefficients, adjustments had to be made for the individual head losses at the transitions. As the sketches in part III show, a schematic method was devised using a generalised head loss term. Thus, for example, the separate head losses at the flow divider, the canal fork and the syphon, were replaced by a single head loss applied to the syphon. The adjustment consisted in reproducing the natural water surface under steady flow conditions. Table 2 compares the natural flow profiles with those obtained from the computer and scale models under steady flow conditions. The adjustment of the two models can be assessed from these results. With the mathematical model thus adjusted, 4 earlier scale model tests on translation waves were put through the computer. Table 3 gives the test conditions. The results obtained on both models in each test, at various points in the canal, are given in figures 5-8. Lastly, a prototype test, carried out on 15th September 1964 in the first reach of the Oraison canal, was reproduced on each model. Figure 9 compares the computed levels with those recorded on the prototype and the scale model. The comparison is satisfactory on both models, except during a short period just after turbine shutdown at about 10 a.m. This is due to the fact that neither model was designed to allow for the effect of the Oraison power station tailrace tunnel. The conclusion is that the computer model results are quite comparable with those yielded by a distorted scale model. The use of mathematical models in preference to distorted scale models would seem to be technically justified in all cases where it is not overruled by economic or other considerations