In this paper, we discuss a general method for symmetry synthesis of electromagnetic multiports. Two approaches are possible in this problem: (1) solution for all the possible symmetry groups describing the multiports, and (2) determination of the symmetry group of a given ideal scattering matrix and calculation of the real matrix. A three-way nonreciprocal divider synthesis illustrates the second approach. Also, for four-ports defined is the group tree allowing one to calculate all the possible scattering matrices.

A new varistor system of SnO2-Bi2O3-Nb2O5 was reported in this paper. Theelectrical field-current density characteristics of this system were investigated by doping different amounts of Bi2O3 and sintering the samples at various temperatures. It is found thatadding 0.75 mol% Bi2O3 to Nb-doped SnO2 ceramic resulted in maximum nonlinear coefficientand breakdown voltage with α = 14 and E 0.5 = 19 525 V/cm. To improve the density as well asthe nonlinearity of this system, different amounts of Co2O3 were added. The optimal conditionsfor the best nonlinearity were 1300 °C with 0.03 mol% Co2O3 addition. Deviation fromthis doping content, toward either higher or lower Co2O3 content, causes the deterioration of I−Vcharacteristics. It can be concluded that the incorporation of cobalt oxides into SnO2-based varistors improves the nonlinearity in the low and intermediate current density regionsbecause of the increased barrier height $(\Phi_{\rm B})$ . The experimental results were explainedwith the defect barrier model for SnO2-based varistors.

p-type (100) faceted diamond films can be successfully grown by bubbling H2 through liquid B(OCH3)3 during the Microwave Plasma Enhanced Chemical Vapour Deposition (MPCVD). Ramann spectra and scanning electron micrographs (SEM) convincingly illustrate that diamond-film growth on silicon substrates by a three-step process yields good uniformity with preferential orientation. The field emission current density of the boron doped diamond films can be enhanced from 0.7 μA/cm2 for the as grown films to 140 μA/cm2 at an applied field of 20 V/μm by hydrogenation treatment.

A large number of lead-free BaTiO3-based ceramics were prepared. Depending on the composition, some of them exhibit a relaxor behaviour whose characteristics were related to the type of ionic substitution and to the substitution rate. The relaxor effect is all the more favoured as the composition deviates more from BaTiO3 and as the substitution is heterovalent in the 6-coordination number crystallographic site. Some of these materials could prove valuable (dielectrics for capacitors and actuators) because they are environment-friendly.

The mixed sulphate acid Rb0.7(NH4)0.3HSO4 presents at room temperature a phase with a structure similar to the one of the parent compounds RbHSO4 and NH4HSO4. Different techniques of the study of phase transitions show that this compound exhibits five phases with varying temperature and in particular the appearance of a new phase (noted IV) in comparison of the phase diagram of parent compounds, this phase takes place in the range of temperature T between 330 and 435 K. The symmetry of the mixed compound at room temperature (phase III) has been performed again by X-ray diffraction on single crystals. The crystallographic characterization of the phase IV is made by X-ray diffraction on powder sample. Crystallographic parameters in this phase are then obtained whereas group theory considerations gives the most probable space group of this phase. In this work, the phase transitions are studied by dielectric measurements. Theoretical study of the behaviour of dielectric susceptibility based on LANDAU theory of phase transitions shows an improper ferroic character of the transition occurring at 330 K with a faintness index f equal to 2.

The transmittance (T) of thin films of CdI2, prepared by thermal evaporation technique on quartz substrates,have been measured over the wavelength range 200−900 nm. Fromanalysis of the transmittance data, the optical constants, therefractive index (n) and the extinction coefficient (k), havebeen studied. Analysis of the refractive index (n) yields a lowfrequency dielectric constant, average oscillator strength and average oscillator energy. From analysis of the absorptioncoefficient (α), the fundamental absorption edge can bedetermined. Both allowed direct transitions and allowed indirecttransitions are observed. The composition of films is checkedusing energy dispersive X-ray (EDX) spectroscopy technique. X-raydiffraction (XRD) measurements showed that the CdI2 filmsevaporated at room temperature substrates were characterized by a polycrystalline form. At large thicknesses the films indicated the inhomogeneity. The effect of annealing temperature (up to 523 K) onthe film properties has been studied.

A comparative nonlinear characterization is carried out between the nonlinear behavior of a composite and the piezoceramic used to obtain it. This study is necessary for using the composite in power transducer applications, as in the Non Destructive Testing (NDT) by ultrasonic waves at high frequencies (300 kHz−800 kHz). Measurements of the losses and the resonator stiffness variations have also been done. Both these effects, as well as the possibility of the frequency hysteresis, show different behavior in the composites, since the increases in the ceramics are different from those in the composites. In this study two measure methods are used; principally the motional impedance increase with the motional current measurements. The results obtained are normalized in order to make them independent of the resonator size, and thus make the comparison between the composite and the ceramic easier. The figure of the mechanical loss tangent tg $\delta_{\rm m}$versus the mean strain $\langle S \rangle $ shows that the losses can be greater in the ceramic than in the composite for soft ceramics. The dependence behavior of the losses versus the mean strain shows that hard composite transducers have lower losses and higher strains than the soft ones. However, these losses in hard composites are higher than those in the bulk ceramic transducers.

Thin amorphous and polycrystalline tungsten oxide films have been prepared by Chemical Vapor Deposition (CVD) from metallorganic precursor − tungsten hexacarbonyl − at atmospheric pressure. The dependence of the composition and the structure of tungsten oxide films on the technological conditions has been investigated by XPS, XRD, DTA-TGA and Raman spectroscopy. As a result it has been established that: at high values of the flow-rates of the reaction gases amorphous films of very low density have been obtained; in the XPS spectra of the understoichiometric WO3−y (0 < y < 0.3) films besides W6+, also W5+ and W4+ stateshave been observed. First to observe in the Raman spectra of amorphous CVD-WO3 films is the band at ~ 950 cm−1,characteristic for terminalW6+ = O bonds in result of the presence of structural water. Theexistence of structural water in the amorphous material has been established by thermal analyze, also.

We study the optical behavior of Au-TiO2 thin cermet films, the gold volume fraction of which is varied over the range 10−3 to 8 × 10−2. In this low concentration range, it is mainly governed by the metallic particles size, which very much depends on the deposition substrate temperature and the annealing time and temperature. The samples structure and morphology are characterized by several complementary techniques. Spectrophotometric measurements at quasi-normal incidence have been performed in visible and near infrared regions. Using a short pulse Nd:YAG laser, we also look for nonlinear optical behavior due to the enhancement of the electromagnetic field close to the surface plasmon resonance of the metallic grains.

This paper proposes a generalised inverse learning structure to control the LCL converter. A feedforward neural network is trained to act as an inverse model of the LCL converter then both are cascaded such that the composed system results in an identity mapping between desired response and the LCL output voltage. Using the large signal model, we analyse the transient output response of the controlled LCL converter in the case of large variation of the load. The simulation results show the efficiency of using neural networks to regulate the LCL converter.

We report on the annealing effect on the structural and magnetic properties of Ta5nm/Cu5nm/FeMn8nm/Co8nm/FeMn8nm/Ta5nm samples prepared by sputtering, on silicon substrate, at room temperature. For the as-deposited samples the interfacial exchange anisotropy field (He) between the Co and FeMn layers is about 95 Oe. It increases sensitively with annealing to reach a maximum value of 135 Oe at 240 °C annealing temperature. Above this temperature, we observe a strong decrease of He, down to 90 Oe after annealing at 280 °C, and an increase of 50% of the saturation magnetization. In order to understand the effect of the annealing on the magnetic properties, we performed a detailed structural analysis by means of Rutherford back scattering spectroscopy (RBS), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM). All these techniques attribute the large changes in the magnetic properties to the formation of a new ferromagnetic FeMnCo phase at the FeMn/Co interfaces.

In this paper, a claw pole permanent magnet machine is studied using a 3D scalar potential formulation. First, the discretisation of Maxwell's equations using Whitney's elements is presented. Especially, the method to take the winding current distribution into account by means of source fields is detailed. Then, a numerical model of the studied machine is presented. Simulation results (inductance, EMF, torque) are discussedby comparison with experimental ones.

This paper presents a modelling of a Transverse Vernier Hybrid Reluctance Motor (TVHRM). The geometry of the magnetic circuit of this latter is especially 3D. The use of the 3D finite element method to do the analysis and to predetermine the motor characteristics is more accurate, but with an important computational time which makes unpractical a parametric study. An analytical model which will allow a rapid investigation of the motor characteristics is a good compromise between accurate results and a weakly simulation time. This is done through two comparable models based both on the equivalent reluctance network method. These analytical models are compared to the 2D finite element one. Finally the predicted characteristics are estimated with the practical test results.

Pb(Zr,Ti)O3 (PZT) and Pb(Zr,Ti,Nb)O3 (PNZT) thin films have been grown on platinized silicon substrates by RF magnetron sputtering followed by a post-annealing treatment. The niobium, Nb, concentration varied from 1 to 7 at.% by increment of 1 at.% The effects of the Nb introduction on the PZT electrical properties, i.e., dielectric and ferroelectric ones have been investigated. We have found that the relative dielectric constant $(\varepsilon_{\rm r})$ is very sensitive to the Nb introduction; $(\varepsilon_{\rm r})$ reaches 1100 for a PNZT film doped at 2 at.% in comparison to 820 for a PZT film. The ferroelectric properties are also dependent of the doping level; in particular the remnant polarization reaches its maximum value ($17 \mu$C/cm2) for a 2 at.% Nb doped PZT film.

We propose to model the spectral response of p-n junction photodetectors based on gallium nitride and related AlGaN alloys. The model is based on the resolution of the differential equations that govern the excess carrier variation in each layer of the photodiode taking into account all the physical parameters, in particular the presence of deep trap levels in the forbidden gap. We notice that the theoretical results are in good agreement with the experiments. We have also analysed the effect of the recombination velocity at the illuminated surface, as well as the impact of the thickness and the doping density of the p-type layer (illuminated zone) on the spectral response magnitude.

We study gallium arsenide (GaAs) high voltage photoconductive switches triggered by a 30 ps neodymium: yttrium aluminium garnet (Nd:YAG) laser, i.e. using photons of energy smaller than the band gap. We measure optical absorption at Brewster incidence under optical pulse excitation and determine the extrinsic one photon and intrinsic two photon absorption coefficients. Analyzing the photoconductive resistance under different power densities of laser radiation, and using the absorption data, we demonstrate that only about 20% of the photons absorbed by the extrinsic process are converted into free electrons. We conclude that high efficiencies can only be obtained by using two photon absorption, which is feasible with ultrafast lasers and focussed beams.

Back-diffusion of electrons to cathode is studied by Monte Carlo simulation for realistic argon cross sections. In particular we study the influence of different aspects of back-diffusion modeling with an aim to simplify the models used in modeling of plasma displays, low pressure gas breakdown and detectors of high energy particles. It was found that the initial electron energy distribution is one of the critical parameters and affects the calculated escape factors very much. The same is true for reflection while angular distribution of initial electrons has a very small influence on the escape factors. The model of cross sections combined with the selection of realistic initial conditions was shown to represent the back-diffusion in argon very well giving good agreement with the available experimental data. Most importantly it was found that the range of electrons returning to the cathode exceeds by far a mean free path and that the number of collisions that they make before returning is quite large. Thus it was found that for a relatively high pressure of around 10 torr the range exceeds d= 1 cm (at E/N=12 Td, 1 Td=$10^{-21} {\rm V}{\rm m}^{2})$ and therefore application of the escape ratios below that value of p d (where p is the pressure) is questionable, i.e. under those conditions calculations should be performed for the actual geometry.

Chromium-doped silica-based optical fibres emit infrared fluorescence at 77 K near 1250 nm (500 nm band-width) under 860−980 nm excitation. Visible and near-infrared absorption spectra of fibres were analysed using the Tanabe-Sugano formalism. It is shown that although the fibre core is codoped with only 1 mol% of aluminium, chromium is preferentially stabilized as Cr4+ in Al-rich regions of the glassy matrix in distorted tetrahedrally coordinated sites. The infrared fluorescence is assigned to Cr4+ along a transition from the 3T2 state down to the 3A2 ground state.

In this paper we present an experimental study concerning electrode erosion under the sole action of an electric arc. An experimental device allowing to generate electric arc current pulses of constant intensity has been realized. It allows also to ignite the arc for a given electrode gap up to 10 mm by means of a HV discharge. The influence of several parameters on the electrode erosion has then been studied. These parameters are: the nature of the contact material (Ag, AgSnO2, AgCdO, Cu, CuSnO2), the value of the electrode gap, the nature of the cover gas... Films realized with the help of a high speed framing camera have brought interesting elements of interpretation concerning the test erosion results.

Within the framework of the study of the aircraft structural material lightning, we present a work concerning the heating of metal sheets under the action of a moving electric arc. A 2D and 3D modeling of thermal phenomena occurring in the heated electrodes are used in order to study the influence of the arc root velocity and of the power and the surface power density brought by the arc roots on the electrode heating.