Mohamed Lamjed Bouazizi

Multi-modal vibration energy harvesting approach based on nonlinear oscillator arrays under magnetic levitation

We propose a multi-modal vibration energy harvesting approach based on arrays of coupled levitated magnets. The equations of motion which include the magnetic nonlinearity and the electromagnetic damping are solved using the harmonic balance method coupled with the asymptotic numerical method. A multi-objective optimization procedure is introduced and performed using a Non-dominated Sorting Genetic Algorithm (NSGA) for the cases of small magnet arrays in order to select the optimal solutions in term of performances by bringing the eigenmodes close to each other in terms of frequencies and amplitudes. Thanks to the nonlinear coupling and the modal interactions even for only three coupled magnets, the proposed method enable harvesting the vibration energy in the operating frequency range of 4.6 − 14.5 Hz, with a bandwidth of 190 % and a normalized power of 20.2 mW cm −3 g −2 .

Use of Metamodels in the Multi-Objective Optimization of Mechanical Structures with Uncertainties

In this paper, we propose a method which takes into account the propagation of uncertainties in the finite element models in a multi-objective optimization procedure. This method is based on the coupling of the Stochastic Response Surface Method (SRSM) and a genetic algorithm of NSGA type (Non-dominated Sorting Genetic Algorithm). The SRSM is based on the use of Stochastic Finite Element Method (SFEM) via the use of the perturbation method. Thus, we can avoid the use of Monte Carlo simulation, whose cost is prohibitive in the optimization problems, especially when the finite element models are large and with a considerable number of design parameters. The objective of this study is, on the one hand, to quantify efficaciously the effects of these uncertainties on the variability of responses which we wish to optimize, and on the other hand, to calculate solutions which are both optimal and robust resulting from the numerical simulation. At the end of a multi-objective optimization procedure, the space of optimal solutions is generally of a large dimension. The solutions obtained are practically non-exploitable by the designer. To facilitate this interpretation, a study of sensitivity a posteriori can be exploited in order to eliminate the non-significant design parameters. The use of the clusters resulting from the Self-Organizing Maps of Kohonen (SOM) is also suggested for a rational management of the design space. The importance of the methodology that we have used along with suggestions for its performances are highlighted by two numerical examples. The criterion of quality selected consists in obtaining the best compromise: the minimal computing time versus the maximum precision of results.

Synthesis, structural and complex impedance spectroscopy studies of Ni0.4Co0.4Mg0.2Fe2O4 spinel ferrite

ABSTRACT Spinel ferrite having composition Ni0.4Co0.4Mg0.2Fe2O4 was prepared by sol-gel method. X-ray diffraction result indicates that the ferrite sample has a cubic spinel type structure. FT-IR showed two absorption bands (ν1 and ν2) that are attributed to the stretching vibration of tetrahedral and octahedral sites. Complex impedance properties have been investigated in 200–420 K temperature range with varying frequency between 40 and 107 Hz. Frequency and temperature dependency of imaginary part of permittivity (ϵ″) and dielectric loss (tanδ) has been discussed in terms of hopping of charge carriers between Fe2+ and Fe3+ ions. Activation energy has been estimated from both temperature dependency of dc conductivity and relaxation time data, which indicates that the relaxation process and conductivity have the same origin. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results.

Nonlinear 2-DOFs Vibration Energy Harvester Based on Magnetic Levitation

The nonlinear dynamics of a two-degree-of-freedom (2-DOFs) vibrating energy harvester (VEH) based on magnetic levitation is modeled and investigated. The equations of motion have been derived while taking into account the magnetic nonlinearity and the electro-magnetic damping. The associated linear eigenvalue problem has been analyzed and optimality conditions have been expressed in term of distance minimization between the two eigenfrequencies of the considered system. The resulting optimal design parameters have been substituted into the coupled nonlinear equations of motion which have been numerically solved. It is shown that the performances of a classical single degree of freedom VEH can be significantly enhanced up to 270 % in term of power density, up to 34 % in term of frequency bandwidth and up to 10 % in term of resonance frequency attenuation.

Above room temperature complex impedance analysis of properties of La0.33Sr0.67Mn0.33Ti0.67O3±δ perovskite

ABSTRACT La0.33Sr0.67Mn0.33Ti0.67O3±δ perovskite was prepared by the standard solid state reaction at 1773 K. The sample crystallizes in the cubic structure. The electrical properties of the sample have been investigated above room temperature in 293–373 K range with varying frequency between 102 and 106 Hz using complex impedance analysis. The sample exhibit a metal–semiconductor transition temperature at TMS = 303 K. The conductance curves were well fitted by Jonscher power law G(ω) = Gdc + Aωn, which shows that with increasing temperature, the exponent (n) decreases below TMS, and increases above TMS. The activation energy deduced from the analysis of the conductance curves matches very well with the value estimated from the relaxation time indicating that relaxation process and electrical conductivity are attributed to the same defect. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results.

Nonlinear dynamics of magnetically coupled beams for multi-modal vibration energy harvesting

We investigate the nonlinear dynamics of magnetically coupled beams for multi-modal vibration energy harvesting. A multi-physics model for the proposed device is developed taking into account geometric and magnetic nonlinearities. The coupled nonlinear equations of motion are solved using the Galerkin discretization coupled with the harmonic balance method and the asymptotic numerical method. Several numerical simulations have been performed showing that the expected performances of the proposed vibration energy harvester are significantly promising with up to 130 % in term of bandwidth and up to 60 μWcm-3g-2 in term of normalized harvested power.

Microstructural, magnetic and electrical properties of Zn0.4M0.3Co0.3Fe2O4 (M = Ni and Cu) ferrites synthesized by sol–gel method

A comparative study of microstructural, magnetic and electrical properties of Zn0.4Ni0.3Co0.3Fe2O4 (ZNCFO) and Zn0.4Cu0.3Co0.3Fe2O4 (ZCCFO) ferrites prepared using sol–gel method have been investigated in this work. X-ray diffraction results indicate that samples have cubic spinel type structure (

Inhomogeneous Wave Correlation for Propagation Parameters Identification in Presence of Uncertainties

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