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International audience; In this paper, the intermittent control is proposed for switched reluctance machine drive to both increase the efficiency and decrease the cost of the traction system for low-cost electric vehicles. With the purpose of maintaining the average machine torque required by the load, the intermittent control imposes a higher phase torque during a shorter time. The simulation and experimental results achieved with three different switched reluctance machines show that the intermittent control increases the system efficiency by reducing the converter losses and the core losses. Additionally, three different strategies of the control have been proposed to analyze their impact automotive application based on the criteria of efficiency, machine vibration and torque pulsation. It can also be easily applied for any switched reluctance machines using average-torque-control-based methods.

We have previously reported how semiconducting and amorphous Y-Ba-Cu-O (a-YBCO) can perform fast and sensitive infrared pyroelectric detection. We recall first these performances for planar and trilayer a- YBCO structures. We then consider migration to THz sensing by using a planar antenna. To minimize the antenna to device impedance mismatch, a- YBCO THz conductivity was measured. The conductivity, obtained by parallel plate waveguide time-domain spectroscopy, is 50 to 150 S/m (0.2 to 1.2 THz range). Antenna structures are suggested for both planar and trilayer devices.

It is generally taken for granted that the bulk minority carrier lifetime in uncompensated phosphorus-doped silicon (Si) wafers is stable under illumination. In contradiction with this long-time belief, we report on a fast and significant lifetime degradation under light soaking for Czochralski wafers sampled from the top ingot end. We demonstrate that defect formation also proceeds during carrier injection by forward biasing Si heterojunction solar cells made from such wafers, suggesting a carrier-induced degradation rather than merely light-induced. The involved bulk defect is shown to preferentially form at the wafer center, where double Thermal Donors are in high concentration. The role of Thermal Donors is investigated further and the obtained preliminary results support their involvement in the reported carrier-induced degradation.

A multiscale model for the behaviour of ferroelectric polycrystalline materials under electro-mechanical loading is proposed. It is based on an energetic description of the equilibrium at the single crystal scale using a statistical estimate of the ferroelectric domain structure. A self-consistent scheme is then used to establish the behaviour of polycrystalline materials. The approach is anhysteretic but hysteresis effects can be added afterwards so as to obtain butterfly ferroelectric loops. It is applied to a tetragonal Lead Zirconate Titanate (PZT). The model allows the investigation of crystallographic texture effects on the overall behaviour and provides an estimate of internal stresses within the material. By way of an example a {100} fibre texture is predicted to generate as much as 150% more longitudinal strain and 33% more electric induction at 1 MV/m compared to an isotropic polycrystal. © 2014 The Authors. Published by Elsevier Ltd.

LGEP 2014 ID = 1513; International audience; The knowledge of superconducting filaments magnetization is an important issue of the LHC main magnets design during the construction at CERN. A new method for modeling a superconducting strand, which consists of several concentric layers of the filaments, is proposed in this paper. The numerical results obtained using the finite-element method demonstrate the coupling and decoupling behaviors between the superconducting filaments via the resistive matrix. The comparison with the earlier work on the superconductor modeling is presented and discussed.

International audience; This paper presents a modeling of thin sheets. An interface condition based on analytical solution is used to avoid a fine mesh. This condition is integrated in a time-domain discontinuous Galerkin method to evaluate the shielding effectiveness. This approach is validated by a comparison with analytical solution. 2-D and 3-D cavities are simulated to illustrate the efficiency of the condition.

International audience; Due to mass constraints, composite materials are possible candidates to replace metal alloys for electromagnetic shielding applications. The design of standard metallic shielding enclosures often relies on finite-element calculations. But in the case of composite materials, the strong dependence on the shielding properties to the microstructure makes the finite-element approach almost impossible. Indeed meshing the microstructure would imply a huge number of elements, incompatible with usual computational resources. We propose in this paper to develop homogenization tools to define the effective electromagnetic properties of composite materials at microwave frequencies. The ratio between the characteristic size of the microstructure and the wavelength is shown to be a key parameter in the homogenization process. The effective properties can then be used as an input for electromagnetic compatibility standard tools, designed for homogeneous media.

We present a hybrid tri-axes magnetometer designed to measure weak magnetic fields in space from DC (direct current) up to a few kHz with a better sensitivity than fluxgate magnetometers at frequencies above a few Hz. This magnetometer combines a wire-wound ferromagnetic ribbon and a classical induction sensor. The nature of the wire-wound ferromagnetic ribbon sensor, giant magneto-impedance or magneto-inductance, is discussed. New configurations of wire-wound ferromagnetic ribbon sensors based on closed magnetic circuits are suggested and the hybrid sensor is described. The electronic conditioning of the wire-wound ribbon makes use of an alternating bias field to cancel the offset and linearize the output. Finally we summarize the main performances of the hybrid magnetometer and we discuss its advantages and drawbacks. A prototype has been built and was part of the scientific payload of the NASA rocket experiment CHARM-II (Correlation of High Frequency and Auroral Roar Measurements) launched in the auroral ionosphere. Unfortunately the launch campaign ended without any noticeable magnetic event and the rocket was eventually launched on 16 February 2010, through a very quiescent arc in the magnetic cusp and no wave activity was detected at frequencies observable by the hybrid magnetometer.

International audience; The aim of this paper is to investigate the glow discharge in its diffuse and filamentary regime, and to study its transition to arc in argon at 100 mbar. The structure of the discharge is observed in correlation with its electrical signals in both static and in dynamic modes. Glow discharges were identified with a positive column that can be fully diffuse, fully filamentary, or a mix of the two with the filamentary part of the column always attached to the anode. Spontaneous transitions between glow discharges and arcs have also been observed and their dynamics are studied. Using high-speed imaging, the transition between glow and arc discharge was identified as the occurrence of two distinct phenomena: a propagation mechanism in the positive column and a constriction of the cathode root. An evaluation of the duration of each phenomenon as a function of electrode gap and current intensity was obtained. The duration of the constriction of the cathode root is found to be on the order of tens of nanoseconds and the duration of the propagation mechanism in the range 50-550 ns.

International audience; In this paper, planar printed-circuit board (PCB) transformers with different winding arrangements are presented. Three different arrangements of the primary and the secondary meander-type windings are designed and fabricated. The electrical parameters of the transformers for all three designs are analyzed and compared. A significant increase of inductance of the primary and the secondary winding is achieved with an appropriate meander-type winding arrangement. In the frequency range of interest, the inductance is approximately 145 nH for the first design, whereas the inductance is approximately 1250 nH for the second and the third design. The coupling coefficient is 0.14 for the first design, whereas it is around 0.9 for the second and the third design. The transformer of the proposed design is to be used in low-power DC/DC converters; therefore, the core with high permeability Ferroxcube 3C90 was chosen. In addition, the transformers of the same winding arrangements were tested with Ferroxcube 3F3 core.

International audience; The use of composite materials for electromagnetic shielding applications contributes to the effort of structure lightening in aerospace industry. In these materials the strong interaction between the electromagnetic field and the microstructure makes the standard numerical tools difficult to implement. Indeed these methods would involve an excessive number of degrees of freedom to describe details of the microstructure. An efficient way to overcome this problem is the use of homogenization techniques providing the effective properties of heterogeneous materials. These effective properties can then be introduced in standard numerical tools to estimate the behavior of shielding enclosures. A recent paper proposes an extension to microwave frequencies of quasistatic homogenization methods. It introduces a characteristic length for the microstructure in the case of a square array of circular 2-D conductive phases embedded in a dielectric matrix. In this paper, a method to identify this length parameter is proposed for random microstructures.

International audience; The magnetic permeability of magnetic materials highly depends on mechanical stress. Stress state is usually multiaxial and has a significant effect on the performance of electromagnetic devices. In this paper a three-parameter analytical model for the stress dependent permeability of magnetic materials is proposed, based on a simplified energetic description of magneto-elastic behaviour. The proposed approach also provides a new equivalent stress for magnetic behaviour in the low-field and low-stress regime.

International audience; The key constituent of silicon heterojunction solar cells, the amorphous silicon/crystalline silicon heterojunction (a-Si:H/c-Si), offers a high open-circuit voltage (Voc) potential providing that both the interface defect passivation and the band bending in the c-Si absorber are sufficient. We detail here analytical calculations of the equilibrium band bending in c-Si (ψc-Si) in Transparent Conductive Oxide (TCO)/a-Si:H emitter/c-Si absorber structures. We studied the variation of some electronic parameters (density of states, work function) according to relevant experimental values. This study introduces a discussion on the optimization of the doped emitter layer in relation with the work function of the TCO. In particular, we argue on the advantage of having a highly defective (p)a-Si:H emitter layer that maximizes ψc-Si and reduces the influence of the TCO on Voc.

International audience; In this paper a new field of application of the overlapping finite element method is proposed for eddy current testing. This method, which has already been used to deal with the lift-off, is extended to the treatment of thin conducting and/or magnetic layers.

In this paper we first present a comparison between two grating techniques, the oscillating photocarrier grating (OPG) and the moving grating technique (MGT). The common origin of both techniques suggests that they should give the same experimental data. In this work we present measurements performed with both techniques and show their equivalence. Regarding the fact that the OPG method uses a lock-in amplifier to measure the photocurrent, while the MGT uses an electrometer due to its dc nature, we show that these techniques can be used in a complementary way, using the benefits that each one of them offers. Moreover, we present measurements of the density of states (DOS) of an amorphous silicon sample, which were made using a range of temperatures that are only achievable using both techniques. We also present a brief summary of the theory supporting these techniques, and we outline future research to improve the DOS estimation. Fil: Ventosinos, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina Fil: Longeaud, C.. Ecole Superieure D; Francia Fil: Schmidt, Javier Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química; Argentina

International audience; In this paper we present new 3D numerical model to calculate the vertical and the guidance forces in high temperature superconductors taking into account the influence of the flux creep phenomena. In the suggested numerical model, we adopt a new approach of the control volume method. This approach is based on the use of an unstructured grid which can be used to model more complex geometries. A comparison of the control volume method results with experiments verifies the validity of this approach and the proposed numerical model. Based on this model, the levitation force's relaxation at different temperatures was also studied.

International audience; Conducting Probe Atomic Force Microscopy (CP-AFM) has been used to perform mechanical and electrical experiments on graphene layers bonded to polyaminophenylene (PAP) films grafted on gold substrates. This technique is a new approach for the characterization of graphene sheets and represents a complementary tool to Raman spectroscopy. The combination of friction and electrical imaging reveals that different stacked graphene sheets have been successfully distinguished from each other and from the underlying PAP films. Lateral Force Microscopy has shown that the friction is greatly reduced on graphene sheets in comparison with the organic coating. The electrical resistance images show very different local conduction properties which can be linked to the number of underlying graphene sheets. The resistance decreases very slowly when the normal load increases. Current-voltage curves display characteristics of metal-molecule-metal junctions.

Using home-built experimental setups, electrical properties and electromechanical characterization of two systems based on multiwalled carbon nanotubes (MWNTs) were investigated at room temperature. The first system is formed by carbon nanotubes (CNTs) either isolated or in small groups on a gold substrate, while the second one concerns a macroscale three-dimensional entanglement of CNTs in powder form. The local electrical resistance on systems of the first type was measured using an atomic force microscopy with a conductive tip and showed a narrow distribution of resistance values as well for isolated CNTs as for small groups of them. However, in this latter case the average resistance value has been found to be one order of magnitude higher than that of individual CNTs, which was attributed to the contact resistance between CNTs. This parameter was then studied from a statistical viewpoint through electromechanical tests performed at a macroscopic scale. They consisted in applying an external compression to CNTs powder samples and measuring the evolution of the electrical resistance across the pressed material. These tests demonstrated an outstanding decrease of the electrical resistance resulting from the increasing number of random connections between CNTs under compression, and the experimental curves were fitted with an analytical model. Furthermore, it was deduced from this model that the elementary contact resistance between CNTs decreases under compression. The stability of this electrical contact was verified over several durations and under different constant applied loads.

LGEP 2011 ID = 753; International audience; In this paper, different electrical characterization techniques are used to investigate the metastability behavior of the ZnO:Al/ZnMgO/Zn(S,O,OH)/Cu(In,Ga)Se2 solar cell. Whereas this device presents a poor conversion efficiency and metastable behavior just after the deposition stage, Thermal Annealing and Light Soaking considerably improved its conversion efficiency. In order to understand the metastable behavior and Thermal Annealing effect of this device, different electrical and electro-optical characterization measurements were carried out including Sub Gap Modulated Photo-Current Spectroscopy. The application of Sub Gap Modulated Photo-Current Spectroscopy measurements performed on an as-deposited device, revealed two deep defects. These defects became shallower and less efficient for the capture of free carriers after Thermal Annealing. Moreover, the photoluminescence spectra exhibit an increase of the measured signal after Thermal Annealing, which is due to a band-to-band recombination. In addition, capacitance-voltage measurements show a hysteresis behavior that is significantly reduced after Light Soaking or Thermal Annealing. We also performed numerical simulations with SCAPS, a software tool of the University of Gent available to the PV research community, to calculate the current-voltage characteristics under AM1.5G illumination and the capacitance-voltage ones under dark conditions. The evolution of the corresponding experimental characteristics from the as-deposited state to different steps during the Thermal Annealing or Light Soaking posttreatment could be well reproduced by varying a high acceptor-like defect density at the buffer/absorber hetero-interface.

LGEP 2011 ID = 952; International audience; In this paper, we numerically study the influence of the speed variation of a magnetic source on the distribution of current density, magnetization, and dissipated energy of a high-temperature superconducting cylinder described by a Jn power law. The results presented come from the resolution of a nonlinear diffusion problem of electric field by a mixed finite-element finite-volume discretization method. This method is robust, stable, and converges for large values of n. The calculations carried out for n, varying from 1 to 200, show that when the external magnetic field quickly varies from 0 to its maximal value, the maximum values of penetration, the magnetization, and the energy dissipation are obtained when the switching of magnetic field occurs. For a periodic magnetic field, we note that any change of the period results in variation of the magnetization and the dissipated energy.

LGEP 2011 ID = 752; International audience; In this work, we investigate the influence of gallium content on the defects properties of co-evaporated CuInxGa1 − xSe2 by subgapmodulatedphotocurrentspectroscopy and admittancespectroscopy techniques. A series of CuInxGa1 − xSe2basedsolarcells with different gallium content in the range from 0% to 33%, and with the same CdS buffer layer have been investigated. On one hand, photocurrentspectroscopy results show 2 types of defects named D1 and D2, and on the other hand, admittancespectroscopy results exhibit only one type of defect. I-V curves show that one of the two defects probed by photocurrent is responsible of the dominant recombination mechanisms next to the heterointerface, between the absorber and the buffer layers. Moreover, I-V curves under AM 1.5 conditions show that the cell with no probed D2 defect presents the best photovoltaic performances.

LGEP 2011 ID = 842; International audience; A main limitation of most models describing the effect of stress on the magnetic behavior is that they are restricted to uniaxial - tensile or compressive - stress. Nevertheless, stress is multiaxial in most of industrial applications. An idea to overcome the strong limitation of models is to define a fictive uniaxial stress, the equivalentstress, that would change the magnetic behavior in a similar manner than a multiaxial stress. A first definition of equivalentstress, called the deviatoric equivalentstress, is proposed. It is based on an equivalence in magneto-elastic energy. This formulation is first derived for isotropic materials under specific assumptions. An extension to orthotropic media under disoriented magneto-mechanical loading is made. A new equivalentstress expression, called generalized equivalentstress, is then proposed. It is based on an equivalence in magnetization. Inverse identification of equivalentstress is made possible thanks to a strong simplification of the description of the material seen as an assembly of elementary magnetic domains. It is shown that this second proposal is a generalization of the deviatoric expression. Equivalentstress proposals are compared to former proposals and validated using experimental results carried out on an iron-cobalt sheet submitted to biaxial mechanical loading. These results are compared to the predictions obtained thanks to the equivalentstress formulations. The generalized equivalentstress is shown to be a tool able to foresee the magnetic behavior of a large panel of materials submitted to multiaxial stress.

In this paper, we show how to use hexahedral and prismatic elements for the efficient development of uniaxial anisotropic dispersive media (UPML) around an initial unstructured tetrahedral primal mesh using the generalized finite-difference (GFD) method. The initial mesh is assumed to have plane external boundaries. Taking into account the properties of the UPML medium, we build a special discrete Hodge in order to have a simple scheme for the time-domain solution of Maxwell's equations. The method is presented for the general 3-D case and numerical results show its efficiency.

A main limitation of most models describing the effect of stress on the magnetic behavior is that they are restricted to uniaxial, tensile or compressive, stress. An idea to overcome this strong limitation is to define a fictive uniaxial stress, the equivalent stress that would affect the magnetic behavior in a similar manner than a multiaxial one. Several authors have tried to define such a criterion. We propose in this paper to compare several equivalent stress definitions, and to apply them in the case of uniaxial and biaxial mechanical loadings for which experimental results are available.

We show that magnetism and elasticity have very similar mathematical structures when fields are considered as differential forms of adequate nature. The same discretization principles and techniques that succeeded in electromagnetism, notably the use of edge elements, then lead to a manageable form of coupled elasto-magnetic problems.

In this paper, the computation of magnetization of a strand constituted of two superconducting square bars immersed in a uniform magnetic field is studied. The numerical results are obtained from the program developed with the FEM. Bean's critical state model is carried out to characterize the electrical behavior law of superconducting materials. The magnetization cycle, the magnetic field and the current density distributions are presented. The influence of strand length on the electromagnetic coupling phenomena in the superconducting square bars is also considered. The obtained results are in good agreement with the theoretical results.

We present a complete theoretical analysis of the modulated photocarrier grating method (MPG). Our calculations show that the response of the sample will be not only a photocurrent modulated with the frequency f of the illumination, but also a response in dc and with frequency 2f. In order to put to test our calculations, we implement experimentally the MPG technique and we apply it to a hydrogenated amorphous silicon sample. We verify the existence of a response at frequency 2f, and we measure the relative intensity of this signal ? which compares well with the result of our theoretical calculation. We also show that under certain conditions the density of states (DOS) at the quasi-Fermi level for the majority carriers is related to the ratio of the MPG and modulated photoconductivity (MPC) signals. We present a simple formula that allows estimating the DOS as a function of energy by changing the temperature and the generation rate. Fil: Schmidt, Javier Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); Argentina Fil: Budini, Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); Argentina Fil: Ventosinos, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico Para la Industria Química (i); Argentina Fil: Longeaud, C.. Universite´s Paris VI et XI; Francia

PurposeThe purpose of this paper is to present an analysis of the force ripples of an open slot permanent magnet linear synchronous motor (PMLSM). A calculation procedure using 2D finite elements method (2D‐FEM) is then evaluated with experimentations.Design/methodology/approachFirst, the studied PMLSM and its main features are introduced. Then, the 2D‐FEM model used to study the motor is presented. The methods used to calculate the force and the meshing procedures are also highlighted. The calculated no‐load force is compared to measurements. Lastly, the validated model is used to study the influence of the current magnitude on the force ripples at load.FindingsIn addition to the no‐load case, the influence of the current magnitude on these forces is presented.Originality/valueThe paper is orientated with a sound industrial background. For that reason, the impact of the current saturation on the thrust generation is presented via the evolution of the thrust coefficient, which is the force to the RMS currents ratio.

In this work we investigate the electronic properties of Glass/Mo/Cu(In,Ga)Se 2 (CIGS)/CdS/ZnO solar cells where the co-evaporated CIGS surfaces are subjected to a two steps chemical treatment. This two steps treatment consisting of a strong oxidative etch (bromine) of the absorber surface followed by a cyanide treatment leads to a significant (by 30%) improvement of the photovoltaic performances. The bromine etching time is used as variable in the study, while admittance spectroscopy is employed for electrically active defects characterization. Particular attention was given to the interpretation of the origin of the defects probed in the samples. One type of defects with activation energy ranging between 0.2 eV to 0.5 eV, commonly probed in the CIGS based devices and labelled N 2 , are passivated by the cyanide treatment. This allowed us to propose a new interpretation concerning the nature of this type of defect.

We have selected stress-annealed nanocrystalline FeSiBCuNb ribbons for trilayer ferromagnetic/copper/ferromagnetic sensors exhibiting high magneto-impedance (MI) ratio. Longitudinal MI δZ reaches 450% at 60 kHz. Measurements of responses of the GMI sensor on the small amplitude ac fields displays sensitivity of 60 V/T up to a few 100 Hz. The noise equivalent magnetic induction is equal to 13 nT/Hz +1/2 , limited to the added noise from electronics.

Different methods have been proposed to use modulated photoconductivity (MPC) measurements in order to extract information about the density of states (DOS) within the gap of defective semiconductors. Depending on the frequency of the modulation, two regimes have to be considered: the high frequency (HF) and the low frequency (LF) regimes. In this paper, we use computer-generated data, obtained from the complete solution of the MPC equations, to test the different procedures proposed to treat the MPC data in both regimes. We show that Bru¨ggemann’s method provides an accurate reconstruction of the introduced DOS provided the capture coefficients are known, while in the LF limit of Kounavis’ method a factor of two is missing. We also test the accuracy of different procedures proposed to extract the capture coefficients of the defects, which are necessary to get absolute DOS values in the methods that utilize the HF regime. The LF–MPC method, on the other hand, has the advantage that the capture coefficients are not needed. Fil: Schmidt, Javier Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Longeaud, C.. Universite Pierre et Marie Curie; Francia Fil: Koropecki, Roberto Roman. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Arce, Roberto Delio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Kleider, J.. Universite Pierre et Marie Curie; Francia

The prediction of the magnetic and magnetostrictive behavior of grain-oriented (GO) silicon steels is discussed. An experimental procedure for the measurement of the magneto-mechanical quantities is first detailed. Experimental measurements of the anhysteretic magnetization and magnetostriction are compared to results from the literature. A multiscale model, based on an energetic approach and infinite medium hypothesis, is used. Significant discrepancies between experiments and predictions are highlighted. The specimen shape combined to large grain size induces some strong boundary (surface) conditions leading to a change in the definition of the local potential energy. A specific demagnetizing term is introduced in the definition of the potential energy, creating an initial heterogeneous distribution of the magnetic domains and saturating the magnetostriction along the rolling direction. This modification strongly increases the ability of the model to predict the magneto-mechanical behavior of GO steels.

This paper addresses a multiscale strategy for the prediction of anhysteretic magneto-elastic behavior and its application to the definition of a magneto-elastic constitutive law for Terfenol-D. The multiscale modeling is based on an energetic procedure at the single crystal scale. Localization and homogenization procedures are then applied to deduce the constitutive law of polycrystalline media from the behavior of the corresponding single crystal. The method is applied first to define the magneto-elastic behavior of single crystals, and the application to polycrystalline samples is then considered. Modeling results are compared to experimental data.

PurposeTo provide a model that allows testing and understanding special damping techniques.Design/methodology/approachThe finite element modeling takes into account the piezoelectric coupling. It is used with a non linear electrical circuit. The approach leads to an accurate tool to observe the behavior of the non linear damping techniques such as synchronized switch damping.FindingsThe model has been validated by comparison with Ansys® but the CPU time required for the model is around one hundred times shorter.Research limitations/implicationsThe proposed model is 1D and the assumptions to use it are not verified for all structures.Practical implicationsThe authors obtain a useful tool for the design of damping structures (for example to find the best localisation of the piezoelectric patches and to test electrical circuits).Originality/valueThe model is used for the design and conception of damping as well as for harvesting structures.

While renewable energies are achieving parity around the globe, efforts to reach higher solar cell efficiencies becomes ever more difficult as they approach the limiting efficiency. The so-called third generation concepts attempt to break this limit through a combination of novel physical processes and new materials and concepts in organic and inorganic systems. Some examples of semi-empirical modelling in the field are reviewed, in particular for multispectral solar cells on silicon (french ANR project MULTISOLSI). Their achievements are outlined, and the limits of these approaches shown. This introduces the main topic of this contribution, which is the use of multiscale experimental and theoretical techniques to go beyond the semi-empirical understanding of these systems. This approach has already led to great advances at modelling which have led to modelling software which is widely known. Yet a survey of the topic reveals a fragmentation of efforts across disciplines, firstly, such as organic and inorganic fields, but also between the high efficiency concepts such as hot carrier cells and intermediate band concepts. We show how this obstacle to the resolution of practical research obstacles may be lifted by inter-disciplinary cooperation across length scales, and across experimental and theoretical fields, and finally across materials systems. We present a European COST Action MultiscaleSolar kicking off in early 2015 which brings together experimental and theoretical partners in order to develop multiscale research in organic and inorganic materials. The goal of this defragmentation and interdisciplinary collaboration is to develop understanding across length scales which will enable the full potential of third generation concepts to be evaluated in practise, for societal and industrial applications., Comment: Draft paper accompanying a plenary presentation to the World Renewable Energy Conference WREC 2015, June 2015, Bucharest. In press (IOP)

The capabilities and limitations of the well-known C–V technique for the determination of the conduction band offsets in (n)a-Si:H/(p)c-Si heterojunctions are presented. In particular, the effects due to the presence of an inversion layer in c-Si and a non-negligible defect density at the a-Si:H/c-Si interface on the reliability of the C–V intercept method are discussed. The influence of the Fermi level positions in (p)c-Si and (n)a-Si:H on the inversion layer formation and the influence of the interface defect density have been analysed using numerical simulations and experimental measurements.

The knowledge of the absorber band gap of heterojunction solar cells is of crucial importance, because it helps to investigate the band offsets at the buffer/absorber heterointerface. Usually this gap is deduced from optical techniques such as Spectral Photo Response (SPR). However, for inhomogeneous materials, this method of estimating the band gap is dominated by the lowest value somewhere inside the absorber layer, and SPR alone cannot give any indication about the spatial location of the region concerned with this measured value. Nevertheless, to determine the nature of band offsets at the heterointerface, we need a more accurate and local value of the gap next to the interface. In this study, dark I–V–T measurements were performed to characterize the gap within the CuIn(S,Se)2 (CISS) Space Charge Region (SCR) near the CdS buffer layer. Under forward bias conditions, several recombination mechanisms occur in the CdS/CISS solar cells, each of them being characterised by a specific activation energy. One of these activation energies is related to the gap of the CISS SCR near the buffer/absorber heterointerface. We present a detailed study of the gap of electrodeposited CISS, determined from I–V–T measurements and its evolution with various CdS deposition conditions. The obtained gap values are found to be 200 to 300 meV higher than those derived from SPR. We explain this difference by the occurrence of graded CISS gap, with a gap value next to the heterointerface higher than the bulk gap value. © 2007 Elsevier B.V. All rights reserved.

As the reliability of electrical microcontacts has proved to be the main limitation to a fast-growing production of ultraminiaturized switches, a thorough understanding of their failure mechanisms is an all-important purpose. This paper aims at showing that conducting-probe Atomic Force Microscopy (cp-AFM) is an adequate tool to actuate and study electrical contacts. By choosing relevant cantilevers and operating mode of the cp-AFM, dimensions, gap and force level representative of existing microelectromechanical switches (MEMS switches) are obtained. With two examples, the advantages of using a cp-AFM in force mode for studying physical phenomena at very low scale are highlighted. The reported investigations concern material transfer between contact parts and contact bounces. Those two undesirable phenomena induce surface damages and impinge reliability of MEMS switches. In both cases an explanatory scenario of phenomena occurring at nanoscale is proposed and preventive recommendations for improving the lifetime of such devices are suggested.

Hopping electrical transport has been investigated in amorphous carbon nitride in both DC and AC modes, as a function of temperature T, electric field F and frequency. The strong field enhancement of the current for F> 5 x 10 4 V cm -1 , causing a departure from the Mott law at low field, i.e. the linear dependence of ln(σ OHMIC ) vs (T 0 /T) 1/4 , is also observed in the AC conductance data. Both conductance and equivalent parallel capacitance are discussed in the framework of a universal model of AC transport with spatially randomly distributed energy barriers. The inverse characteristic time 1/τ, which also follows Mott's law, is compatible with the filling rate Γ F (E DL ) of empty states at the transport energy, which governs the DC conductivity.

This paper presents an original method for the modeling of microcoils at high frequency dedicated for different applications: radio frequency, nuclear magnetic resonance, nondestructive testing, etc. The aim of the modeling is to determine the elements of an electric equivalent circuit. A magneto-dynamic three-dimensional (3-D) finite-element formulation is used to calculate the resistance and the inductance of the microcoil at high frequency. Using the previous results, an electrostatic 3-D finite-element field analysis is then used to determine the capacitance. The method was tested by using an example of nuclear magnetic resonance microcoil. Numerical results obtained by the proposed method are compared with measurements and analytical results.

Deposition of hydrogenated amorphous silicon (a-Si:H) thin films by rf-PECVD from a silane highly diluted into hydrogen mixture was shown to give more stable layers and solar devices. However, hydrogen–silane mixture is quite hazardous. That is why the possibility to obtain stable and device grade thin films using argon dilution of silane has been investigated. We present the correlations we found between transport, structural and deposition plasma properties of a-Si:H films prepared from silane–argon mixtures, these correlations leading to the choice of optimum deposition parameters resulted in device grade silicon thin films. We also present the results obtained on a large set (75 nos.) of 1 cm2 p–i–n diodes for which all the layers were deposited under argon dilution including the p and n layers. A mean conversion efficiency of 6.5% was reached, the best devices exhibiting conversion efficiency of 8%. The light degradation is smaller than with diodes prepared with conventional a-Si:H and the best diodes present a 7% stabilized efficiency after 100 h under AM1.5 illumination. We conclude that argon dilution of silane can be an alternative to the hydrogen dilution to prepare high conversion efficiency and stable devices by rf-PECVD with the major asset of a less hazardous process.

The temperature and electric field dependence of the current density in hydrogenated amorphous carbon and carbon nitride sandwich devices show a T−1/4 dependence of the ohmic conductivity σ=σ00·exp[−(T0/T)1/4] characteristic of 3D hopping, a linear correlation between the apparent Lnσ00(F) and T0(F) values, and a F/T3/2 scaling of σ(F,T). The F2 dependence of Lnσ(F,T) at intermediate fields, provides similar values of the decay length γ−1 of localized π state wavefunctions in a-CNx:H (2.4 nm) and a-C:H (2.8 nm) films. The larger conductivity in nitrogen-rich alloys is attributed to a combined effect of N-induced increase in the π-state density and ordering of the sp2 phase.

Polycrystalline CVD diamond is a very attractive material for the fabrication of UV detectors. However, good quality material is required to obtain a high UV photoresponse together with a low visible sensitivity. In this work, we present and correlate the results of Raman spectroscopy, spectral photoresponse and thermally stimulated current (TSC) achieved on polycrystalline CVD diamond films. All the layers show the typical Raman diamond line with very small FWHM values, a broad TSC peak in the 480–530 K range, and have a high UV-visible selectivity (defined here as the ratio of the photosensitivity at 200 and 657 nm). This selectivity is found to increase when the FWHM of the Raman line decreases. In the same time, the mobility-lifetime product increases, and we observe both a shift in the TSC peak from 530 to 480 K and a decrease in the integrated trapped charge. For the best sample, the FWHM of the Raman line is 1.88 cm−1, and the values of the mobility-lifetime product and of the UV-visible selectivity are as high as 1.5×104 cm2 V−1, and 2×107, respectively, while the trap density estimated from the TSC experiment is in the range of 1017 cm−3.

International audience; Metal-substituted phthalocyanine (MPc) thin films as zinc- or copper-phthalocyanine are often used as charge injection layers for organic electroluminescent or photovoltaic devices. It is then important to characterize their electronic defect density and band structure near their gap. In this work the monolayer structures were prepared by vacuum sublimation of the organic thin film sandwiched between indium tin oxide (ITO) and aluminum electrodes. Electrically active defects were investigated with space-charge capacitance spectroscopy, as a function of temperature and frequency, in the range 80–330 K and 40 Hz to 10 MHz, respectively. Organic materials are best described on the basis of individual molecular orbital (HOMO and LUMO) energies instead of valence and conduction band. Such energies were derived from cyclic voltammetry and optical absorption spectroscopy measurements. Experimental results were correlated to the electrical J–V characteristics of these MPc based devices to gain more insight on the charge injection processes and their limitations.

The theoretical background of the Schottky barrier capacitance of disordered semiconductors is recalled and the temperature, frequency, and bias dependence of the capacitance is illustrated on hydrogenated amorphous silicon (a-Si:H), hydrogenated polymorphous silicon (pm-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H). Both a-Si:H and pm-Si:H follow the same trends, with a lower defect density in pm-Si:H. Quite different features are observed in μc-Si:H, and are related to different defect levels and to the two-phase constitution of the material. Finally, the capacitance analysis is reviewed in the framework of the amphoteric dangling bond states, where the influence of both defect transitions is emphasized.

International audience; The III–V semiconductor materials provide a range of opto-electronic properties well suited to bandgap engineering and high efficiency solar cells. The design process, III–V growth and fabrication methods are described for homogeneous and heterogeneous structures, and the magnitude of fundamental thermal and radiative losses for important III–V solar cell materials calculated. An analytical model is presented, analysing solar cell performance in detailed terms of processes in the space charge region and charge neutral layers of solar cells. The model formulates the solar cell radiative efficiency as a function of bias, providing a quantitative measure of how close devices come to the ideal efficiency limit. Single junction pin and record efficiency pn GaAs cells are analysed and their radiative efficiency quantified, concluding that radiatively dominated behaviour is reached in the more efficient np design. Tandem and triple junction III–V concepts are reviewed and efficiency limits placed in the context of achievable designs. Experimental data are modelled for both structures and the radiative efficiency quantified. The more radiatively efficient tandem design is found to be closer to its fundamental efficiency limit for a radiatively dominated dual junction structure, as a consequence of lower non-radiative recombination rates. The application of III–V materials to quantum confined structures is finally reviewed with specific regard to the quantum well solar cell and its demonstrated 90% radiative efficiency at high bias as a result of the lower bandgap undoped multiple quantum well region.

LGEP 2014 ID = 1593; International audience; The effect of stainless steel cathode surface morphology on field electronic emission in high vacuum (~10⁻₅ Pa) is studied. The surface rugosity is shown to affect the emission intensity; high-aspect ratio surface features lead to locally enhanced electric field, thereby increasing the field electronic emission. A breakdown in the vacuum, caused either by impact of charged dust particles or other impurities, or by overheating and vaporization of field emitters, can lead to cratering on the cathode surface. This cratering drastically changes the local surface rugosity, enhancing the local field and leads to greater emission intensity. Treatment of the cathode surface by glow discharge at higher pressure is known to reduce field emission by ionic bombardment and sputtering of emission sites; an image of such a glow-type discharge in the same apparatus used to study field emission is presented.

LGEP 2014 ID = 1589; International audience; This paper deals with temperature measurements of the surface of tungsten electrodes submitted to electric arcs in nonstationary regimes in air at atmospheric pressure and with the estimation of the power and power density brought by electric arcs to the electrodes. Temperature measurements are proposed for tungsten anodes and cathodes for current intensity of ~30 A and duration of 3 ms. The temperature of the electrode surface at the instant the arc extinguishes is estimated in both electrode polarities. These experimental results are used in a model to estimate the power and the power density brought to the tungsten electrode surface by electric arcs. We found that the power surface density brought by electric arcs to the electrodes was in the range 1. × 109-1.5 × 109 W/m2 in the case of anodes and in the range 1.3×109-1.9×109 W/m2 in the case of cathodes. It has also been found that reactions of tungsten oxidation had to be considered to explain the high values obtained for the power brought to the electrodes.

International audience; This work deals with the characterization of DC electric arcs in aeronautical conditions of pressure (104 Pa–105 Pa). Characteristics of electric arcs such as the mean electric field in the arc column, the anode and cathode voltage drops, the length for which the arc extinguishes naturally and the mean energy dissipated per arc have been measured for three current values: 7, 70 and 300 A. The decrease of pressure leads to an increase in the arc length which can be two times longer in pressure conditions reached during a flight than in atmospheric pressure conditions. This is correlated with the variation of the mean electric field with pressure. Consequently, the energy dissipated per arc is also amplified.

Internal stresses (residual stresses) and their evolution under electromechanical loading play a significant role in the performance and lifetime of ferroelectric materials. Internal stresses can be induced by the manufacturing process or the packaging conditions. At a finer scale, due to the heterogeneity of ferroelectric materials (polycrystalline structure), part of the ferroelectric strain must normally be accommodated locally, resulting in internal stresses when an electromechanical load is applied. In most modelling approaches the contribution of crystalline elastic anisotropy to internal stresses is neglected compared to the domain switching contribution. Using a micromechanical modelling approach, we examine the contribution of crystalline elastic anisotropy to the macroscopic behaviour and to the internal stress distribution in ferroelectric polycrystals under electromechanical loading. It is shown that the predicted macroscopic ferroelectric strain and the level of intergranular stress can be underestimated by up to 50% if crystalline elastic anisotropy is neglected. Consequently it is recommended that, contrary to current practice, crystalline anisotropy be included in the micromechanical modelling of ferroelectric ceramics. © 2014 IOP Publishing Ltd.