A new varistor system of SnO2·Ni2O3·Nb2O5 exhibits the relative dissatisfactory physical and electrical properties. The effect of cobalt oxide on the properties of the SnO2·Ni2O3·Nb2O5 varistors was investigated by measuring the densities, permittivities, the current-voltage properties and the properties of the defect barriers. It is found that the sample doped with 0.25 mol% Co2O3 exhibits the abnormal poorer electrical properties than the samples without Co2O3 dopants. However, the sample doped with 6.0 mol% Co2O3 exhibits the highest nonlinear coefficient ( $\alpha=17.24$ ) and reference electrical field ( $E_{\rm B}=561$ V/mm), although the sample doped with 1.0 mol% Co2O3 exhibits the highest densities ( $\rho =6.87$ g/cm3) and permittivities. The sample with 6.0 mol% Co2O3 bears the highest barriers, but the sample with 1.0 mol% Co2O3 shows the narrowest barriers. The investigation of the sintering temperature shows that the samples sintered at 1450 °C exhibit the better physical and electrical properties. The results were discussed and the defect barrier model was also introduced to explain.

PbO-As2O3 glasses containing different concentrations of Cr2O3 (ranging from 0 to 0.4 mol% ) were prepared. The differential thermal analysis, IR andoptical absorption spectral studies of these glasses were carried out; from these studies,the variations in the values of glass transition and crystallization temperatures, opticalband gap and the shifting of symmetrical bending vibrational frequency band were observed with increase in concentration of Cr2O3. Magnetic susceptibilities for these glasses were also determined and Cr3+ ion concentration was estimated. In addition, detailed studies on dielectric properties viz., dielectric constant ε, loss tan δ, ac conductivity $\sigma_{\rm ac}$ and dielectric breakdown strength were investigated. Using the results of these studies, the structural modifications taking place in the glass network were discussed in the light of Cr3+ ion concentration.

This paper presents a new method of switching management for four quadrant switches (FQS) of matrix converters. By discriminating the FQS command orders, it is possible to lay down the FQS configuration (transistor or diode). The presented method uses the load current sign and zero closeness information to ensure good sequencing of command orders. By using Mealy state machines, we simplify the existing state diagrams for any phase number switching cell. The successful results on a three-phase switching cell, without oversized power circuit, validate this new state diagram structure.

We report the recent progress in the structuration of nanoporous silicon at a submicronic scale. Multilayered periodic structures, with low-roughness interfaces, and an index contrast of 0.6 are reported. Their strong photonic bandgap properties are demonstrated by the measurement of a reflectance up to 99.64% ± 0.01%, by mean of ring-down spectroscopy.This property is the base of a novel structure allowing an efficient guiding of light in alow-index layer. The submicronic structuration of the optical index of a planar waveguidealong one direction of plane is also described, and a realistic map of the optical index,demonstrating a index contrast of 0.5, is unambiguously deduced from transmission spectra.

We report on characterisation of optical modes in InP microcavities formed by two-dimensional (2D) photonic crystals (PC) on a suspended membrane. Diffracted photoluminescence (PL) experiments and numerical calculations are associated in order to analyse the properties of PC microresonators and waveguides and to investigate the coupling between these two building blocks of PC based photonic integrated circuits.

We have developed a model of calculation of the induced current due to an electron beam. The expression for the electron beam induced current (EBIC) with an extended generation profile is obtained via the resolution of a steady state continuity equation by the Green function method, satisfying appropriated boundary conditions to the physical model. The generation profile takes into account the lateral diffusion, the effect of defects, dislocations and recombination surfaces besides the number of absorbed electrons and that of diffuse electrons as a function of the depth. In the case of a Schottky diode Au/GaAs obtained bymetalorganic vapour phase epitaxy (MOVPE) method, the theoretical induced current profile iscompared to the experimental one and to theoretical profiles whose analytical expressionsare given by van Roosbroeck and Bresse. The minority carriers diffusionlength Ln = 2 µm and the optical self-absorption coefficient a = 0.034 µm−1 can be deduced from the experimental current profile, measured byscanning electron microscopy. The theoretical curve, obtained from the proposed model is ina good agreement with the experimental one for surface recombination velocity106 cm s−1 except for distances far from the depletion layer (x 0 >2.3 µm)where the photocurrent produced by the multiple process of the reabsorbed recombinationradiation is preponderant. Our results are in agreement with those obtained by otherexperimental techniques on the same samples.

We have developed a model for the calculation of the induced current due to an electron beam with an extended generation profile. The analytical expression of the electron beam induced current (EBIC) is obtained by solving the steady-state continuity equation using the Green function method.In the case of a sulphur doped (Ga0.7Al0.3As:N+/Ga0.7Al0.3As:P) sample prepared by metalorganic vapour phase epitaxy (MOVPE) method, the experimental current profile, measured by SEM enabled us to calculate the diffusion length of the minority carriers (Lp = 1 μm in the N region and Ln = 1.80 μm in the P region of the ternary sample). Far from the depletion layer, the experimental current profile measured provided us the optical self absorption coefficient of this sample: ap = 1.483 μm−1 in the N region and an = 0.167 μm−1 in the P region. According to our EBIC model, the width of the depletion layer of this sample is about 0.8 μm, while at elaboration of the sample, 10 years ago, the width of the depletion layer deduced from the characteristic curve I(V) was about 300−400 Å. This widening of the depletion layer is due to the ageing of the diode.

The aim of the present work is to provide data to understand better and quantify the dielectric strength of pure compressed N2 (1 < P < 8 bar). The lack of agreement of results found in the literature leads us to study the electrical and physical characteristics of this gas as a function of a wide set of parameters. Experiments are mainly performed in point to plane nitrogen gaps up to 40 mm length using three different rod radii. Gas conditioning phenomenon in positive polarity and corresponding change in characteristics of discharge mechanism are demonstrated here. In our experimental conditions, no leader is involved in the discharge development whatever the polarity is. The values of the breakdown reduced field $E_{\rm b}/P$ are determined as a function of the pressure. Two different experiments are performed in order to take into account the mechanisms of initiatory electron production: the influence of the γ-ray irradiated N2 and that of the surface roughness of the HV electrode. In negative polarity, the free electrons are provided from the cathode whereas they are provided from the gas under positive polarity. Voltage waveform influences on the U 50 breakdown voltage are dealt with in a large pressure range (1 < P < 12 bar). The role of the non-uniformity of the applied electric field on the U 50 value is pointed out in both polarities. Finally, the reduced guiding field of negative streamer is experimentally measured.

We investigate the effects of geometrical micro-irregularitieson the conversion efficiency of reactive flows in narrowchannels of millimetric size. Three-dimensional simulations, based upon aLattice-Boltzmann-Lax-Wendroff code, indicate that periodic micro-barriers may have an appreciable effect on the effective reaction efficiency of the device. Once extrapolated to macroscopic scales, these effects canresult in a sizeable increase of the overall reaction efficiency.