Graphene–Mesoporous Si Nanocomposite as a Compliant Substrate for Heteroepitaxy

The ultimate performance of a solid state device is limited by the restricted number of crystalline substrates that are available for epitaxial growth. As a result, only a small fraction of semiconductors are usable. This study describes a novel concept for a tunable compliant substrate for epitaxy, based on a graphene–porous silicon nanocomposite, which extends the range of available lattice constants for epitaxial semiconductor alloys. The presence of graphene and its effect on the strain of the porous layer lattice parameter are discussed in detail and new remarkable properties are demonstrated. These include thermal stability up to 900 °C, lattice tuning up to 0.9 % mismatch, and compliance under stress for virtual substrate thicknesses of several micrometers. A theoretical model is proposed to define the compliant substrate design rules. These advances lay the foundation for the fabrication of a compliant substrate that could unlock the lattice constant restrictions for defect‐free new epitaxial semiconductor alloys and devices.     

Adaptive Nonlinear Control of Wind Energy Conversion System Involving Induction Generator

This paper presents a theoretical framework for adaptive control of a wind energy conversion system (WECS), involving a squirrel cage induction generator (SIG) connected with an AC/DC/AC IGBT‐based PWM converter. A multi‐loop nonlinear controller is designed to meet two main control objectives, i.e., (i) speed reference optimization in order to extract a maximum wind energy whatever the wind speed, and (ii) power factor correction (PFC) to avoid net harmonic pollution. These objectives must be achieved despite the mechanical parameters uncertainty. First, a nonlinear model of the whole controlled system is developed within the Park coordinates. Then, a multi‐loop nonlinear controller is synthesized using the adaptive backstepping design. A formal analysis based on Lyapunov stability is carried out to describe the control system performances. In addition to closed‐loop global asymptotic stability, it is proven that all control objectives (induction generator speed tracking, rotor flux regulation, DC link voltage regulation and unitary power factor) are asymptotically achieved.     

Enhancement of sparse 3D reconstruction using a modified match propagation based on particle swarm optimization

Sparse 3D reconstruction, based on interest points detection and matching, does not allow to obtain a suitable 3D surface reconstruction because of its incapacity to recover a cloud of well distributed 3D points on the surface of objects/scenes. In this work, we present a new approach to retrieve a 3D point cloud that leads to a 3D surface model of quality and in a suitable time. First of all, our method uses the structure from motion approach to retrieve a set of 3D points (which correspond to matched interest points). After that, we proposed an algorithm, based on the match propagation and the use of particle swarm optimization (PSO), which significantly increases the number of matches and to have a regular distribution of these matches. It takes as input the obtained matches, their corresponding 3D points and the camera parameters. Afterwards, at each time, a match of best ZNCC value is selected and a set of these neighboring points is defined. The point corresponding to a neighboring point and its 3D coordinates are recovered by the minimization of a nonlinear cost function by the use of PSO algorithm respecting the constraint of photo-consistency. Experimental results show the feasibility and efficiency of the proposed approach.

     

Temperature and liquid crystal concentration effect on thermal conductivity of poly(styrene) dispersed 5CB liquid crystal

In the present work, we study the thermal behavior of Polymer (Polystyrene) dispersed (4‐cyano‐4′‐pentylbiphenyl) 5CB liquid crystal film composite. A photopyroelectric device was used to study thermal conductivity at homeotropic and planar aligned of 4‐cyano‐4′‐pentylbiphenyl (5CB) liquid crystal. Thermal conductivity of polystyrene (PS) has been determined and calculated from experimental applied data reported in the literature. Thermal conductivity characteristics of the PDLC films were investigated with three prediction models as a function of both temperature and liquid crystal concentration in the polymer matrix. We particularly show the behavior of this thermal conductivity in the ON and OFF state. It was found that the difference in the film thermal conductivity ranges between 3 and 21%, depending on the ON and OFF state and the liquid crystal volume concentration. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 481–486, 2003     

Eikonal Equation Adaptation on Weighted Graphs: Fast Geometric Diffusion Process for Local and Non-local Image and Data Processing

In this paper we propose an adaptation of the Eikonal equation on weighted graphs, using the framework of Partial difference Equations, and with the motivation of extending this equation’s applications to any discrete data that can be represented by graphs. This adaptation leads to a finite difference equation defined on weighted graphs and a new efficient algorithm for multiple labels simultaneous propagation on graphs, based on such equation. We will show that such approach enables the resolution of many applications in image and high dimensional data processing using a unique framework.     

Crystallization behavior and spherulitic morphology of poly(lactic acid) films induced by casting process

A novel way using a polarized optical microscopy and statistical image analysis techniques for direct investigation of the crystallization kinetics and spherulitic morphology of poly (lactic acid) induced by casting process has been proposed. DSC analyses were carried out to explore the effect of drawing in thermal properties of PLA. We found that the rate of cold crystallization was sharply increased with draw ratio. Isothermal calorimetric results reveal that thin films crystallize slowly compared to sheets. Moreover, thin films don't fit the Avrami model demonstrating that more stretching may modify the crystallization kinetics of films. A comparison was made between pure PLA and stretched materials using Hoffman theory and giving a compiled view on the details of crystallization behavior with relation to stretch effect. At draw ratio 167, the growth rate reach its maximum and the crystallization rate go up exponentially. POM experiments showed the existence of two different spherulitic populations in thin films and two nucleation mechanisms was identified. Accordingly, the growth rate was found to be not linear with time. Furthermore, the X‐ray diffraction analyses showed the presence of α and α^' crystal form in thin films contrarily to sheets with α form. POLYM. ENG. SCI., 55:1881–1888, 2015. © 2014 Society of Plastics Engineers     

Fundamental understanding and modeling of diffuse interphase properties and its role in interfacial flow stability of multilayer polymers

This work aims to investigate the growth and structural evolution of a diffuse interphase generated in a flow field and to highlight its importance on controlling the final properties of compatible multilayer materials. The model polymers chosen are poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF). Interdiffusion kinetics, geometrical, and rheological properties of the interphase decoupled and coupled to flow have been probed and quantified under fundamental conditions of rheological measurement and under real practical conditions of processing, respectively. Polymer chain orientation is shown to decelerate the interdiffusion coefficient. This phenomenon is demonstrated to be balanced by intermixing (i.e. flow effect) at the vicinity of the interface triggered from excess interfacial shear stress, thus favors development of the interphase. The diffuse interphase generated therein during the processing has been well characterized via scanning electron microscopy coupled with energy dispersive X‐ray analysis (SEM‐EDX) and transmission electron microscopy (TEM). Results indicate that the interphase is robust with a geometrical property of tens microns depending on processing conditions and the interfacial structure is shown to be smooth with continuous amorphous‐crystallization transition between neighboring layers without causing any interfacial disturbances. Finally, experimental studies concerning the interfacial flow instability and encapsulation in coextrusion process have been performed, taking into account some key classical decisive parameters such as viscosity ratio, thickness ratio and elasticity ratio, etc. Different from severe flow instability observed in incompatible multilayer systems, presence of the interphase at PMMA/PVDF multilayers plays a vital role in weakening (or even eliminating) the viscous instabilities and elastic instabilities despite of the very high rheological contrast. POLYM. ENG. SCI., 55:771–791, 2015. © 2014 Society of Plastics Engineers     

Evaluation of the dynamic properties of PVC foams under flexural vibrations

The paper presents an investigation of the damping of PVC foams under flexural vibrations of clamped-free beams. The PVC foams are constrained by two aluminium beams and different densities of the PVC foams are studied. An experimental investigation is implemented using an impulse technique. The natural frequencies and the damping of the beams are modelled by using a finite element analysis based on the sandwich theory. Next, the numerical and experimental results are used to derive the shear modulus and the damping of PVC foams as functions of the frequency. Finally, the experimental investigation and the developed modelling show how the damping of aluminium–foam beams must be corrected to estimate the damping of PVC foams.     

Simple and fast convergent procedure to estimate recursive path analysis model

Behaviormetrika - BFGS procedure is classically used for the estimation of the parameters of a recursive Path Analysis model. In practice, BFGS does not present any problem of convergence. However,...