Dielectric properties of colossal permittivity materials: An update

During the last 10 years, a lot of interests have been devoted to the so-called CDC (colossal dielectric constant) materials. The first materials exhibiting this behavior were the perovskite-based ceramics on the CCTO (CaCu3Ti4O12) composition. Relative dielectric permittivity can attain values up to (or even larger than) 105. Nevertheless, their dielectric losses are too high, the lower values ranging 10%, in a narrow frequency range, thus limiting their applications. The underlying physical mechanisms at the origin of the CDC are still under study. The analysis of broadband impedance spectroscopy measurements leads most of the authors to propose an interfacial polarization mechanism (at the electrodes or at internal barriers), there is a limited number of complementary electrical characterization techniques, which, up to now, comfort the proposed interfacial polarization mechanisms. In the present work, I-V and time-domain polarization are used to characterize these materials. One of the main results is the observation of a non-symmetrical response of these materials related to the direction of the polarization. These results are observed for both macroscopic level on bulk polycrystalline material and within individual grains of the same samples. These results do not fit current accepted models for polarization for CDC materials

Metallized ceramic substrate with mesa structure for voltage ramp-up of power modules

International audienceAs the available wide bandgap semiconductors continuingly increase their operating voltages, the electrical insulation used in their packaging is increasingly constrained. More precisely the ceramic substrate, used in demanding applications, represents a key multi-functional element is being in charge of the mechanical support of the metallic track that interconnects the semiconductor chips with the rest of the power system, as well as of electrical insulation and of thermal conduction. In this complex assembly, the electric field enhancement at the triple junction between the ceramic, the metallic track borders and the insulating environment is usually a critical point. When the electrical field at the triple point exceeds the critical value allowed by the insulation system, this hampers the device performance and limits the voltage rating for future systems. The solution proposed here is based on the shape modification of the ceramic substrate by creating a mesa structure (plateau) that holds the metallic tracks in the assembly. A numerical simulation approach is used to optimize the structure. After the elaboration of the structures by ultrasonic machining we observed a significant increase (30%) in the partial discharge detection voltages, at 10 pC sensitivity, in a substrate with a mesa structure when comparing to a conventional metallized ceramic substrate

How Linear Tension Converts to Curvature: Geometric Control of Bone Tissue Growth

This study investigated how substrate geometry influences in-vitro tissue formation at length scales much larger than a single cell. Two-millimetre thick hydroxyapatite plates containing circular pores and semi-circular channels of 0.5 mm radius, mimicking osteons and hemi-osteons respectively, were incubated with MC3T3-E1 cells for 4 weeks. The amount and shape of the tissue formed in the pores, as measured using phase contrast microscopy, depended on the substrate geometry. It was further demonstrated, using a simple geometric model, that the observed curvature-controlled growth can be derived from the assembly of tensile elements on a curved substrate. These tensile elements are cells anchored on distant points of the curved surface, thus creating an actin “chord” by generating tension between the adhesion sites. Such a chord model was used to link the shape of the substrate to cell organisation and tissue patterning. In a pore with a circular cross-section, tissue growth increases the average curvature of the surface, whereas a semi-circular channel tends to be flattened out. Thereby, a single mechanism could describe new tissue growth in both cortical and trabecular bone after resorption due to remodelling. These similarities between in-vitro and in-vivo patterns suggest geometry as an important signal for bone remodelling

Commande diffusive d'une machine électrique : une introduction

On propose, sur l'exemple particulier d'un variateur de vitesse à moteur à courant continu, une voie nouvelle de synthèse de commande robuste, basée sur une nouvelle approche dite ''diffusive''. De manière très schématique, la méthode consiste, à partir d'une loi de commande traditionnelle, d'améliorer cette dernière par l'adjonction d'un ''transfert diffusif'' optimisé. L'objectif de la communication n'est pas de développer cette méthode dans toute sa généralité, mais plutôt d'en présenter la démarche et certains aspects fondamentaux sur un cas simple pour lequel le correcteur diffusif optimal est de type fractionnaire

Modelisation et commande d'un moteur a allumage commande d'automobile

SIGLECNRS T 68488 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Luminescence de l'argon et du xénon dans les décharges à barrières diélectriques mono-filamentaires (analyses électriques et cinétique)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Modélisation comportementale en génie électrique sous représentation diffusive (méthodes et applications)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Identification of dielectric relaxations: a suitable infinite-dimensional state-space approach

International audienceThe concept of Diffusive Representation is used in this paper for modeling purposes in the field of electrical engineering, namely when some non-rational dynamics are involved [2]. Diffusive formulations are convenient for analysis and cheap numerical approximations. We study the problem of time-domain identification of the so-called diffusive symbol η(ξ) associated to the permittivity ε(p) of dielectric material. The so-obtained state-space models allow simple and precise time-domain simulation of such dynamical components