Lattice stability and the effect of Co and Re on the ideal strength of Ni:First-principles study of uniaxial tensile deformation

Using first-principles density functional calculations, lattice stability of γ-Ni under [001], [110], and [111] uniaxial tensions and the effect of alloying elements Co and Re on the uniaxial tensile behavior of γ-Ni were studied in this paper.With elastic constants and phonon spectra calculations, we examined the mechanical stability and phonon stability of Ni during the uniaxial tensions along the three characteristic directions. The results show that the mechanical stability and phonon stability of a lattice occurs before the maximum stress–strain point under the [001] and [111] tension, respectively.The effects of Co and Re on the ideal tensile strength of γ-Ni show a significant directivity: Co and Re have little effect on the stresses in [001] and [111] directions, but increases the ideal strength of the system in the weakest uniaxial tensile direction. Moreover, the strengthening effect of Re is significantly better than that of Co. By further analyzing electronic structure, it is found that the effect of alloying elements on the uniaxial tensile behavior of γ-Ni comes from their interactions with host atoms.     

A high-purity circular polarization modulator: Application to birefringence and circular dichroism measurements on multidielectric mirrors

We present a method for producing a modulated circular polarization of high purity, i.e. whose modulation frequency allows specific discrimination against linear polarization and unpolarized intensity. We apply it to measure the circular dischroism and birefringence of high reflectivity multidielectric mirrors: no circular dichroism is observed at a level of 10^?6, but birefringence values from 3 x 10^?4 to 2 x 10^?3 radian per reflexion under normal incidence are obtained.     

Modeling ferrocene reactions and iron nanoparticle formation: Application to CVD synthesis of carbon nanotubes

This paper presents a practically useful model that can predict the formation process and the growth rate of iron nanoparticles from ferrocene. There is a strong need to create such a model that can help improve the process of carbon nanotube (CNT) production by chemical vapor deposition (CVD) and flame synthesis methods. This study serves this purpose. A simple reaction mechanism between ferrocene and hydrogen was proposed based on experimental and theoretical studies conducted by other researchers; then the method of moments with interpolative closure (MOMIC) was applied to compute the size distribution of iron particles. MOMIC was found to reproduce the result of a sectional method with better accuracy than a lognormal method. We simulated the isothermal decomposition of ferrocene and the consequent formation of iron particles with MOMIC; the computed diameter of iron particles agreed reasonably well with experimental data. Analytical solutions for particle diameter were obtained as a function of residence time, assuming monodisperse particles and a constant collision frequency function. They compared very well to the numerical prediction by a monodisperse model and reasonably well to that by MOMIC, suggesting the analytical solutions to be a simple first approximation in assessing the performance of CVD and flame synthesis methods of producing CNTs.     

Integrated Digital Image Correlation considering gray level and blur variations: Application to distortion measurements of IR camera

The acquisition of images with different modalities may involve different alterations with respect to an ideal model. Inhomogeneous brightness and contrast, blur due to non-ideal focusing, distortions are common. It is proposed herein to account for such effects for instance by registering a calibration target image with an actual optical image to measure lens distortions. An Integrated Digital Image Correlation (I-DIC) algorithm is proposed to account for the above artifacts and the algorithm is detailed. The resolution and uncertainty of the technique are first investigated on synthetic images, and then applied to the measurement of distortions for infrared (IR) images. The procedure is shown to reduce drastically the residual level assessing the validity of the image formation model, but more importantly allowing for a much improved registration of images.     

Adsorption of carbon dioxide and coadsorption of carbon dioxide and hydrogen on iron: I. Volumetric measurements,adsorption isotherms,changes in resistance and work function

The adsorption of CO₂ on thin evaporated iron films is studied at 196, 273, 300 and 370 K by means of volumetric measurements and measurements of equilibrium pressures, changes in work function and electrical resistance. Preadsorbed hydrogen only slightly influences the amount of a consecutive CO₂ adsorption at 273 K, but has a strong influence on the change in resistance due to the CO₂ adsorption. An iron film totally covered with CO₂ at 273 K is able to adsorb further hydrogen. Carbon dioxide and hydrogen seem not to compete for the same adsorption sites.     

Nodal and modal strategies for longitudinal thermal diffusivity profile estimation: Application to the non destructive evaluation of SiC/SiC composites under uniaxial tensile tests

In this paper, infrared image sequences of a SiC/SiC composite excited with a non uniform heat pulse (also known as Flash) are processed using two different strategies. In a first approach, a modal strategy based on the decomposition of infrared sequences in an orthogonal basis is applied. A discrete version of the heat equation is solved in the transformed space using only a very limited number of modes for which the sensitivity to diffusivity is optimal. In a second time, a nodal approach that consists in solving locally a finite difference scheme is performed at each pixel. A new sensitivity analysis based on a local correlation study of the partial derivatives of the experimental data (transformed or not) allows validating the relevance of each approach, and determining the optimal space where to perform estimations. As a result, effective local longitudinal diffusivity profiles of a SiC/SiC composite under mechanical testing are provided and analyzed to detect the presence of microcracks.     

Application of frequency-domain linearized Euler solutions to the prediction of aft fan tones and comparison with experimental measurements on model scale turbofan exhaust nozzles

Although it is widely accepted that aircraft noise needs to be further reduced, there is an equally important, on-going requirement to accurately predict the strengths of all the different aircraft noise sources, not only to ensure that a new aircraft is certifiable and can meet the ever more stringent local airport noise rules but also to prioritize and apply appropriate noise source reduction technologies at the design stage. As the bypass ratio of aircraft engines is increased - in order to reduce fuel consumption, emissions and jet mixing noise - the fan noise that radiates from the bypass exhaust nozzle is becoming one of the loudest engine sources, despite the large areas of acoustically absorptive treatment in the bypass duct. This paper addresses this ‘aft fan’ noise source, in particular the prediction of the propagation of fan noise through the bypass exhaust nozzle/jet exhaust flow and radiation out to the far-field observer. The proposed prediction method is equally applicable to fan tone and fan broadband noise (and also turbine and core noise) but here the method is validated with measured test data using simulated fan tones. The measured data had been previously acquired on two model scale turbofan engine exhausts with bypass and heated core flows typical of those found in a modern high bypass engine, but under static conditions (i.e. no flight simulation). The prediction method is based on frequency-domain solutions of the linearized Euler equations in conjunction with perfectly matched layer equations at the inlet and far-field boundaries using high-order finite differences. The discrete system of equations is inverted by the parallel sparse solver MUMPS. Far-field predictions are carried out by integrating Kirchhoff's formula in frequency domain. In addition to the acoustic modes excited and radiated, some non-acoustic waves within the cold stream-ambient shear layer are also captured by the computations at some flow and excitation frequencies. By extracting phase speed information from the near-field pressure solution, these non-acoustic waves are shown to be convective Kelvin-Helmholtz instability waves. Strouhal numbers computed along the shear layer, based on the local momentum thickness also confirm this in accordance with Michalke's instability criterion for incompressible round jets with a similar shear layer profile. Comparisons of the computed far-field results with the measured acoustic data reveal that, in general, the solver predicts the peak sound levels well when the farfield is dominated by the in-duct target mode (the target mode being the one specified to the in-duct mode generator). Calculations also show that the agreement can be considerably improved when the non-target modes are also included, despite their low in-duct levels. This is due to the fact that each duct mode has its own distinct directionality and a non-target low level mode may become dominant at angles where the higher-level target mode is directionally weak. The overall agreement between the computations and experiment strongly suggests that, at least for the range of mean flows and acoustic conditions considered, the physical aeroacoustic radiation processes are fully captured through the frequency-domain solutions to the linearized Euler equations and hence this could form the basis of a reliable aircraft noise prediction method.     

Calculation of the residual resistivity based on multiple-plane-waves and iterative solutions to the Boltzmann equation: Application to aluminium alloys

We describe a multiple-plane-wave pseudopotential calculation for the residual resistivity in alloys. The electronic wavefunctions, the Fermi surface, and the scattering rates are all determined from a 4-OPW calculation, while the mean free path is determined from an iterative technique which is developed for solving the Boltzmann equation. Numerical results obtained for Al alloys reveal that in many cases the OPW-mixing leads to dramatic enhancement of the residual resistivity over the free electron value, and that this enhancement improves agreement with experiment.     

The two messages of complexity theories and their implications to the study of society: Comments by J. Portugali on the Visioneer white papers by D. Helbing and S. Balietti

This article comments on the Visioneer (Envisioning a Socio Economic Knowledge Collider) Project as described in the following white papers [1–3].^{b b}The comments are based on my new book Complexity, Cognition and the City [4].     

Thermal and field emission effects of laser radiation on metal whisker diodes: Application to infrared detection devices

In a series of recent experiments, research groups have made absolute frequency measurements with laser beams in the infrared region of the spectrum (λ ? 10 μm) using a metal point contact diode for generation, frequency mixing and detection. It has been postulated that the mechanism for the nonlinear current-voltage characteristic of the diode is tunnelling of electrons through an intermediate oxide film from the whisker into the metal base, i.e., the configuration is considered to be a metal-oxide-metal (MOM) tunnelling junction. Several features of the diode's operation create considerable doubt concerning the applicability of the MOM tunnelling mechanism. Analysis of the available experimental data led us to postulate an alternate solid state mechanism, namely a thermally enhanced field emission process. Such emission would be a consequence of the immersion of the whisker tip in the laser radiation resulting in (1) conduction heating which induces thermionic emission and (2) generation of an electric field at the tip necessary for electron tunnelling by field emission. In this paper we calculate rigorously the power absorbed in the metal whisker from the incident radiation. From the power absorbed, the heat conduction equation is solved for model geometries to obtain the laser induced temperature distribution at the whisker surface. Estimates of the electric field are obtained and combined with temperature calculations to obtain the nonlinear I–V characteristics of the thermally enhanced field emission model. Finally some simple experiments are proposed to test the thermal field emission hypothesis as a possible mechanism to explain the nonlinear characteristics of the metal whisker point contact diode.     

Application of van der Waals density functional to an extended system: adsorption of benzene and naphthalene on graphite

It is shown that it is now possible to include van der Waals (vdW) interactions via a nonempirical implementation of density functional (DF) theory to describe the correlation energy in electronic structure calculations on infinite systems of no particular symmetry. The vdW-DF theory [Phys. Rev. Lett. 92, 246401 (2004)] is applied to the adsorption of benzene and naphthalene on an infinite sheet of graphite, as well as the binding between two graphite sheets. A comparison with recent thermal-desorption data [Phys. Rev. B 69, 155406 (2004)] shows great promise for the vdW-DF method.