Short-range order and F ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part II: Al NMR investigation and proposal of clustering process

A ²⁷Al nuclear magnetic resonance (NMR) investigation at room temperature of the Pb_{1 − x}Al_xF_{2 + x} (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb₉Al₂F₂₄ phase is carried out. Two different types of aluminium ions are identified and their ratio is determined as a function of x. A clustering process based on the formation of column clusters, more and more extended when x increases, is proposed on the basis of results issued from the ¹⁹F (Part I) and ²⁷Al NMR investigations.     

Efficient coupling and relaxed alignment tolerances in pigtailing of a laser diode using dual ball lenses

In this paper, we report an efficient coupling scheme with relaxed misalignment tolerances. The proposed coupling scheme consists of two ball lenses of same diameter (1 mm) and different refractive indices. The second ball lens which is facing the fiber tip has a higher refractive index (1.833), whereas the first one which faces the laser diode has a refractive index of 1.5. Employing Gaussian and ABCD ray tracing optics, the theoretically obtained coupling efficiency can reach a unity with relaxed working distance (separation of the coupling system from the fiber tip) in the range between 1 and 4 mm at some optimum positions of the coupling lenses with regard to each other and to the facet of the laser diode. It has been found that if the distance between the first ball lens and the laser diode (d₁) is fixed at 1.1 mm, which is twice its focal length, the coupling efficiency and the working distance as well as the misalignment tolerances are greatly affected by variation of the separation between the two ball lenses (s), and for this proposed coupling scheme the highest coupling efficiency and largest working distance are obtained when s is in the range of 0.3-0.35 mm. Above and below this range there is a significant reduction in the values of the above-mentioned parameters. Experimentally, the Nd:YAG laser welding system has been used for the alignment and welding of the coupling components in a butterfly configuration; the experimentally obtained coupling efficiency of the proposed coupling system was around 75% with relaxed working distance. From the effect of lateral and angular offsets on coupling efficiency, it is clearly noticed that the mode field of laser diode is transformed from elliptical into circular and hence effectively matched with that of the single-mode fiber.     

Investigation on the flameholding mechanisms in supersonic flows: backward-facing step and cavity flameholder

Abstract  

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-ε turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.     

Study of nonlinear system stability using eigenvalue analysis: Gyroscopic motion

General computational multibody system (MBS) algorithms allow for the linearization of the highly nonlinear equations of motion at different points in time in order to obtain the eigenvalue solution. This eigenvalue solution of the linearized equations is often used to shed light on the system stability at different configurations that correspond to different time points. Different MBS algorithms, however, employ different sets of orientation coordinates, such as Euler angles and Euler parameters, which lead to different forms of the dynamic equations of motion. As a consequence, the forms of the linearized equations and the eigenvalue solution obtained strongly depend on the set of orientation coordinates used. This paper addresses this fundamental issue by examining the effect of the use of different orientation parameters on the linearized equations of a gyroscope. The nonlinear equations of motion of the gyroscope are formulated using two different sets of orientation parameters: Euler angles and Euler parameters. In order to obtain a set of linearized equations that can be used to define the eigenvalue solution, the algebraic equations that describe the MBS constraints are systematically eliminated leading to a nonlinear form of the equations of motion expressed in terms of the system degrees of freedom. Because in MBS applications the generalized forces can be highly nonlinear and can depend on the velocities, a state space formulation is used to solve the eigenvalue problem. It is shown in this paper that the independent state equations formulated using Euler angles and Euler parameters lead to different eigenvalue solutions. This solution is also different from the solution obtained using a form of the Newton-Euler matrix equation expressed in terms of the angular accelerations and angular velocities. A time-domain solution of the linearized equations is also presented in order to compare between the solutions obtained using two different sets of orientation parameters and also to shed light on the important issue of using the eigenvalue analysis in the study of MBS stability. The validity of using the eigenvalue analysis based on the linearization of the nonlinear equations of motion in the study of the stability of railroad vehicle systems, which have known critical speeds, is examined. It is shown that such an eigenvalue analysis can lead to wrong conclusions regarding the stability of nonlinear systems.     

Informational analysis of seismic sequences by applying the Fisher Information Measure and the Shannon entropy: An application to the 2004–2010 seismicity of Aswan area (Egypt)

The interevent-time (IET) and interevent-distance (IED) series of seismic events occurred at Aswan area (Egypt) from 2004 to 2010 were investigated by means of the Fisher Information Measure and the Shannon entropy. The analysis was performed varying the depth and the magnitude thresholds. The results point out to an increase of level of organization and order with the decrease of magnitude threshold and the increase of depth threshold for the IET series, while the IED series are characterized by a level of uncertainty approximately constant with the threshold magnitude. The complexity measure, calculated as the product of the Fisher Information Measure and the Shannon entropy power, presents very similar pattern for both the types of seismic series, indicating an increasing complexity with the decrease of the threshold magnitude and the increase of the threshold depth.     

Sulphide based anode material for lithium rechargeable battery

In this study, lead sulphide (PbS) was prepared by the chemical bath deposition technique. The sample was characterized by X-ray diffraction (XRD), Energy Dispersive Analysis of X-rays (EDAX) and cyclic voltammetry. EDAX spectrum shows peaks attributable to lead and sulphur. The EDAX analysis also shows that the prepared sample is stoichiometric. Cyclic voltammetry experiments were recorded at 100 mV·s⁻¹ and 400 mV·s⁻¹ scan rates. Results show that the rate controlling electrochemical reaction is electron transfer. The presence of redox waves shows that the lithium intercalation and deintercalation can occur as a result of lattice expansion in PbS. There were no differences in the PbS XRD data before and after the cyclic voltammetry experiments indicating that the PbS structure is not modified upon lithium ion intercalation and deintercalation in PbS. The discharge characteristics for 35 cycles of the cell using the LiCoO₂/PbS couple is presented indicating the possible development of such materials as anode in lithium ion cells.     

Ag‐doped TiO2 enhanced photocatalytic oxidation of 1,2‐cyclohexanediol

The superiority of silver‐doped titania for photocatalytic oxidation (PCO) of organic compounds inspired us to investigate PCO of 1,2‐cyclohexanediol. Ag/TiO₂ was prepared, characterized (nanosize 19–24 nm) and used for oxidation of 1,2‐cyclohexanediol (1) in acetonitrile. The photolysate was analyzed using GC and GC/MS techniques. The PCO products are 2‐hydroxylcyclohexanone (2), 1,2‐cyclohexandione (3), 2‐cyclohexenone(4), cyclohexanone (5), and adipic acid (6).The formation of electron–hole pair at the surface of the catalyst followed by oxidation reactions was the suggested mechanism. Kinetic studies revealed first‐order mechanism for PCO of 1 and rate constant (k) = ?0.145 h^–1. Copyright © 2012 John Wiley & Sons, Ltd.     

Magnetocaloric properties in the Cr-doped La0.7Sr0.3MnO3 manganites

Single-phase polycrystalline samples of La_0.7Sr_0.3Mn_1-xCr_xO₃ with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature T_C decreases with increasing x and the maximum magnetic entropy change (−ΔS_M) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.