Complex susceptibility analysis of magneto-fluids: Optical band gap and surface studies on the nanomagnetite-based particles

An aqueous stable magnetic fluid containing Fe₃O₄ nano-particles with a mean diameter of 4–7 nm, which is in the range of super-paramagnetism, is prepared. The particles are synthesized via co-precipitation method from ferrous and ferric solutions. X-ray diffraction, transmission electron microscopy, transformer method are used to study the physical properties of the magnetic fluids and powders. A method is given to analyze and resolve the real and imaginary parts of the measured complex susceptibility of magnetic fluids. The band gap parameters of the magneto-nanopowders such as the direct-, indirect-band gap energies, Fermi energy and Urbach energy are determined. A comparative study between the different techniques used to calculate the powder particle size is presented. Adsorption of nitrogen gas is used to identify and determine the particles mean diameter and to study their microstructure, the magnetic properties and surface porosity. The study showed that the total pore system of the magnetic nano-powders consists mainly of mesopores.     

Control of methane flame properties by hydrogen fuel addition: Application to power plant combustion chamber

This study presents a numerical investigation of the effects of mixing methane/hydrogen on turbulent combustion processes taking place in a burner similar to that integrated in gas turbine power plants. Thereby, in comparison to the reference case where the burner is fuelled by 100% of methane, the variations of the axial velocity field, temperature field and mass fraction of carbon monoxide field are examined for different percentages of hydrogen fuel injection. The computed results, obtained by using the software Fluent-CFD, are compared and validated against experimental reference data. Results show that the hydrogen addition to the methane has an impact on all physical and chemical parameters of the reactive system.     

Adaptive Backstepping Control Design for Semi-Active Suspension of Half-Vehicle With Magnetorheological Damper

This paper investigates the problem of controlling half-vehicle semi-active suspension system involving a magnetorheological (MR) damper. This features a hysteretic behavior that is presently captured through the nonlinear Bouc-Wen model. The control objective is to regulate well the heave and the pitch motions of the chassis despite the road irregularities. The difficulty of the control problem lies in the nonlinearity of the system model, the uncertainty of some of its parameters, and the inaccessibility to measurements of the hysteresis internal state variables. Using Lyapunov control design tools, we design two observers to get online estimates of the hysteresis internal states and a stabilizing adaptive state-feedback regulator. The whole adaptive controller is formally shown to meet the desired control objectives. This theoretical result is confirmed by several simulations demonstrating the supremacy of the latter compared to the skyhook control and passive suspension.      

Silane-functionalized Al2O3-modified polyurethane powder coatings: Nonisothermal degradation kinetics and mechanistic insights

This work reports on nonisothermal degradation kinetics of polyurethane (PU)-based powder coatings containing 1, 3, and 5%wt% vinyltrimethoxysilane functionalized Al₂O₃ (V-Al₂O₃) nanoparticles. Thermogravimetric analysis of PU/V-Al₂O₃ powder coatings with different V-Al₂O₃ contents has been performed at different heating rates. Variation of activation energy (E_a) of PU/V-Al₂O₃ powder coatings was modeled as a function of partial mass loss by using Kissinger–Akahira–Sunose, Ozawa–Wall–Flynn and modified Coats–Redfern isoconversional approaches. The results revealed hindered decomposition process of PU/V-Al₂O₃ nanocomposite powder coatings, featured by an increase in activation energy of degradation from ∼158 for blank PU to 225, 183, and 229 kJ/mol for nanocomposites filled with 1, 3, and 5 wt% of V-Al₂O₃, respectively. Likewise, pre-exponential factor values increased for samples containing V-Al₂O₃ nanoparticles compared to that of blank sample. Sestak–Berggren kinetic model appropriately captured thermal degradation behavior of PU/V-Al₂O₃ nanocomposites than that of nth order decomposition kinetic reaction models.     

Passivity and passivity breakdown of zinc anode in alkaline medium

The electrochemical behaviour of zinc in NaOH solutions has been investigated by using potentiodynamic technique and complemented by X-ray analysis. The E/i curves exhibit active, passive and transpassive regions prior to oxygen evolution. The active region displays two anodic peaks. The passivity is due to the Formation of a compact Zn(OH)₂ film on the anode surface. The transpassive region is assigned to the electroformation of ZnO₂. The reverse sweep shows an activation anodic peak and one catholic peak prior to hydrogen evolution. The influence of increasing additives of NaCl, NaBr and Nal on the anodic behaviour of zinc in NaOH solutions has been studied. The halides stimulate the active dissolution of zinc and tend to break down the passive film, leading to pitting corrosion. The aggressiveness of the halide anions towards the stability of the passive film decreases in the order: I^? > Br^? > Cl^?. The susceptibility of zinc anode to pitting corrosion enhances with increasing the halide ion concentration but decreases with increasing both the alkali concentration and the sweep rate.     

Rayonnement parasite des terminaisons court-circuitées en ligne microfente et en guide d’onde coplanaire

An integral equations technique solved by the moment method associated with simple one-port model is used to analyse parasitic radiation of short circuit terminations of slotlines and coplanar waveguides. This parasitic effect is represented by a frequency-dependent normalized resistance. Theoretical results are compared to experimental ones for substrates with several dielectric constants.     

Mélangeur équilibré dans la bande de 26,5 à 40 GHz en technologie de la ligne à ailettes

The fin line technique has been used in a balanced 27.25–29.75 GHz mixer with a 3.75 GHz intermediate frequency. The performance of essential mixer circuits elements is presented. The mixer has an available conversion loss of 5.8 ± 0.8 dB. The mixer is tunable by variable shorts.     

Conception assistée par ordinateur de circuits millimétriques incluant des discontinuités de type ligne à ailettes

Single and complex fin line discontinuities are precisely computed by means of the modal analysis combined with the spectral domain approach. Validity of the results is established by measurements of the scattering parameters of symmetrical configurations including these discontinuities in a «back to back» arrangement. Frequency dependent equivalent networks are derived and they can be used as data base in circuit simulation programs.     

On-line power system security analysis

A broad overview of on-line power system security analysis is provided, with the intent of identifying areas needing additional research and development. Current approaches to state estimation are reviewed and areas needing improvement, such as external system modeling, are discussed. On-line contingency selection has become practical, particularly for static security. Additional work is necessary to identify better indices of power system stress to be used in on-line screening filters for both static and dynamic security analysis. Use of optimal power flow schemes to recommend optimal preventive and corrective strategies is presented on a conceptual level. Techniques must be further developed to provide more practical contingency action plans, which include real-world operating considerations and use a reasonably small number of control actions. Techniques must be developed for costing operating variables which are not easily quantified in dollars. Soft or flexible constraints and time variables must be included in the preventive and corrective strategy formulation. Finally, the area of on-line transient and dynamic security analysis is presented