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Hydrogels with electrically conductive nanomaterials find numerous biomedical applications where conductivity is relevant, such as substrates for tissue engineering of electroactive cells, strain-sensitive sensors and platforms for drug delivery.

A graphene composite material suitable for facile drop-casting into flexible, conductive thin film electrodes from aqueous solution under ambient conditions is reported. This composite was applied to a polyethylene terephthalate substrate obtained from soda bottles. A solution of reduced graphene oxide with few layers (∼6 layers) was synthesized. The composites were prepared in two steps, first applying the reduced graphene oxide with surfactant and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) and then, solutions of poly-3-hexylthiophene with three different concentrations. Mild annealing was required only for the first step. The composite films exhibit more stability (mechanical and electrical) in bending tests when an intermediate amount of poly-3-hexylthiophene is applied. This composite is found attractive for flexible optoelectronic applications, promoting the reuse of one-use plastics.

International audience; A series of three articles presents an innovative way to build advanced functionally graded materials (FGM) based on polymer/ceramic (epoxy/SrTiO3) composites tailored by electrophoresis for field grading in power electronics. In this Part 3, this method is applied in the context of power modules for DBC substrate encapsulation. An evaluation of the FGM performances is reported based on electrostatic simulations and breakdown voltage measurements on encapsulated DBC substrates. The results show a significant mitigation of the electric fringe field at the triple point while breakdown is largely increased by a factor 2 for FGM composites compared to neat epoxy. The process enables to use the electric field reinforcements of HV electrical systems (e.g. tips coming from the design), and thus potential weak points, to locally 'self-heal' them in-situ. Such an electrophoresis process used to build FGM composites paves the way of the next generation of functionalized polymer composites used in high voltage power applications for improving the electrical aging of insulating materials and power system reliability.

International audience; The electrical behaviour of nanocomposite hydrogels and especially hydrogels containing carbon nanotubes is generally poorly understood. In this paper, we investigate the influence of double-wall carbon nanotubes (DWCNT) content on the electrical properties of agarose/DWCNT nanocomposite hydrogels. These nanocomposite hydrogels are potential candidates as electrode materials for transdermal drug delivery by electropermeabilization. Both alternating current (AC) and direct current (DC) measurements at different voltage amplitudes were performed, as well impedance spectroscopy (1 Hz–1 MHz). Data suggest a non-linear dependence of the conduction phenomena vs the applied electric field. From the current-voltage characteristics, the nanocomposite conduction phenomenon is narrowed to two possible mechanisms, a Schottky type or a Poole-Frenkel type. These findings are the first step towards the understanding of the conduction phenomena in such complex nanocomposite structures, comprising DWCNT, water and the 3D polymeric network. The work described in this work is of much wider interest because this kind of nanocomposites may have many other applications, while the fundamental questions about their electrical conductivity remain universal.

International audience; This paper provides a robust experimentalsimulation approach in predicting the Partial discharge inception voltage (PDIV) for aeronautical applications. First, two polymers used as insulators in aeronautic components have been selected in this work, a polyimide and a polyetheretherketone. To study the electrical behavior of these materials, extensive dielectric spectroscopy measurements are carried out for different electric fields, temperatures and frequencies (E, T, and f). Then, a needleplane configuration is adopted for PDIV modeling using finiteelement method (FEM). As the model is strongly dependent upon the accurate estimation of the electric field, dielectric characterizations results are integrated in simulations to improve the precision of calculations. Thereafter, the PDIV can be estimated with the aid of Townsend's theory. Finally, a comparison between measured and computed PDIV values is presented and discussed.

International audience; A series of three articles present an innovative way to build advanced functionally graded materials (FGM) based on polymer/ceramic composites tailored by electrophoresis from the process principle to their field grading application in power electronics. In this Part 2, it was studied the impact of a non-uniform electric field on the high-k SrTiO3 particle organization within an epoxy matrix. In that purpose, DBC substrates with sharp metallization tracks were used to generate a strong electric field divergence. This nonuniform field has been used to organize the particles around the field reinforcement regions during the electrophoresis process. It was discovered that the particles selfarrange into a conformal composite FGM layer that presents a local permittivity gradient: the highest ε being located around the electric field peak areas. This new process could be used to selectively 'heal' the electric field-induced weaknesses of any high voltage electrical system by using its own design.

International audience; A series of three articles present an innovative way to build advanced functionally graded materials (FGM) based on polymer/ceramic composites tailored by electrophoresis from the process principle to their field grading application in power electronics. In this Part 1, the process is presented and relies on applying a DC voltage on liquid composite compound before curing in order to 'freeze' the particles where they have been accumulated. To exemplify this principle, FGM composites involving an epoxy resin with SrTiO3 high-k particles with a permittivity gradient are presented. The methodology to build and characterize them is carried out. An accumulated particle region is observed at the high voltage electrode while the depleted bulk region remains unmodified. This accumulated particle layer both increases in thickness and in filler densification with increasing the field and/or time allowing tuning its permittivity. This work paves the way to the development of more robust electronic systems where the electrical fringe field in critical regions can be mitigated.

One of the main goals in aeronautic industry is to increase the total electrical power in on-board systems. The first approach is to increase the voltage while reducing the volume of converters. However, this approach induces very high electrical constraints on the insulating materials used for power module. These local high electric fields can cause premature failure, by partial discharge activity and insulation breakdown. In order to efficiently reduce the electrical stresses in high voltage power modules, the design of new stress-control encapsulating composite materials with graded properties has been developed using the particles electrophoresis auto assembling technique. It is a promising solution since it does not impact the volume of the power module and has a negligible impact on the overall mass. The aim of this paper is to evaluate different strategies to achieve such field grading encapsulating composite with local high relative permittivity ($\varepsilon_{r} \gt 10)$, tailored around critical areas where the electric field is high. The present paper mainly proposes two processes to enable the electrodeposition of a field grading layer within a composite encapsulation to cover multiple electrodes of a direct bonded copper (DBC) substrate with a complex layout. The first approach consists in adding a new and fixed electrode on the layout of DBC substrate that will not be polarized or used during normal operation of the module. The second method consists in adding a removable top electrode above the substrate. Results show that the latter approach is potentially the best choice to enable the electrodeposition of a homogenous thickness of field graded layer on a complex DBC layout with multiple adjacent copper tracks.

We report a study on the effect of the substitution of Bi3+ by Sr2+ on the stabilization of R3c structure of Bi1−xSrxFeO3 (0 ≤ x ≤ 0.3, Δx = 0.05), and its effect in the magnetic and dielectric behavior. Stoichiometric mixtures of Bi2O3, Fe2O3 and SrO were mixed and milled for 5 h using a ball to powder weight ratio of 10:1 by high-energy ball milling. The obtained powder were pressed at 900 MPa to obtain cylindrical pellets and sintered at 800 °C for 2 h. X-ray diffraction and Rietveld refinement were used to evaluate the effect of Sr2+ on the crystal structure. In addition, vibrating sample magnetometry (VSM) and dielectric tests were used for describing the multiferroic behavior. The results show that Sr-doped BiFeO3 particles present rhombohedral structure (R3c) characteristic of α-BiFeO3 when the doping is below 0.10 mol of Sr. Additionally, a gradual decrease in the amount of secondary phases with the increase of the amount of strontium is observed. For doping concentration higher than 0.15 mol of Sr, a phase transition to an orthorhombic symmetry (β-BiFeO3, Pbnm) is detected. Besides, changes in relative intensities of reflection peaks planes (110) and (104) are associated with the phase transformations and with the magnetic and dielectric behavior. The α-BiFeO3 phase show antiferromagnetic behavior and high values of dielectric permittivity, whereas the β-BiFeO3 phase show a ferromagnetic behavior and low dielectric permittivity.

International audience; Applications of polymeric materials in electrical engineering increasingly require improvements in operating voltages, performance, reliability, and size reduction. However, the resulting increase on the electric field in electrical systems can prevent achieving these goals. Polymer composites, functionalized with conductive or semiconductive particles, can allow us to reduce the electric field, thus grading the field within the system. In this paper, a comprehensive review of field-grading materials, their properties, and recent developments and applications is provided to realize high-performance high-voltage engineering applications.

High voltage power modules are used in numerous applications to build high power converters. Technically, these modules are made of different materials and among them, dielectric materials are organic and inorganic. Organic insulators (gels) are used to avoid corona discharges in the vicinity of connecting wires and high voltage dies (diodes and transistors) and to protect them from moisture and contaminants. Inorganic insulators (ceramic substrates) are used to insulate the high voltage which dies from the grounded elements and to transfer heat to the heat sink. Despite being used since the late 90s, there is a lack of fundamental knowledge about the electrical properties of these substrates. Consequently, manufacturers tend to assure the reliability by over sizing them. As there are no clear rules for how to do that, failures occur, leading to the converter shutdown. The aim of this study is to bring new information about the understanding of the dielectric strength of ceramic materials used in these modules. We have focused our work on the correlation between the mechanical and the dielectric properties of ceramics by using relevant experiments. We provide new information about the impact of existing cracks on the ceramic dielectric failure, according to the electromechanical breakdown model. Our conclusions bring crucial information about the precautions to be taken during manufacturing and implementation of these substrates in power modules to reduce the likelihood of the particular causes of failure.

Multilayer ceramic capacitors (MLCC) are essential components for determining the reliability of electronic components in terms of time to failure. It is known that the reliability of MLCCs depends on their composition, processing, and operating conditions. In this present work, we analyzed the lifetime of three similar X7R type MLCCs based on BaTiO3 by conducting High Accelerated Life Tests (HALT) at temperatures up to 200 &deg, C at 400 V and 600 V. The results were adjusted to an Arrhenius equation, which is a function of the activation energy (Ea) and a voltage stress exponent (n), in order to predict their time to failure. The values of Ea are in the range of 1&ndash, 1.45 eV, which has been reported for the thermal failure and dielectric wear out of BaTiO3-based dielectric capacitors. The stress voltage exponent value was in the range of 4&ndash, 5. Although the Ea can be associated with a failure mechanism, n only gives an indication of the effect of voltage in the tests. It was possible to associate those values with each type of tested MLCC so that their expected life could be estimated in the range of 400&ndash, 600 V.

In power electronics, there is the need to develop solutions to increase the power density of converters. Interleaved multicellular transformers allow interleaving many switching cells and, as a result, a possible increase in the power density. This converter is often composed of a magnetic core having the function of an intercell transformer (ICT) and, depending on the complexity of the designed architecture, its shape could be extremely complex. The switching frequencies (1-10 MHz) for the new wide band gap semiconductors (SiC, GaN) allow to interleave switching cell at higher frequencies than silicon-based semiconductors (

New polymer-based high-permittivity materials often depend on the addition of high-k particles such as BaTiO 3 . In the present work we propose the use of dielectrophoretically-aligned composites with anisotropic properties in the direction of the applied ac field during the curing process. The results on the dielectric permittivity of epoxy-based BaTiO 3 ant SrTiO 3 composites show that permittivity is increased for the anisotropic composites. Nevertheless the dielectric strength of such anisotropic composites is lower than the isotropic ones, possibly due to the increased ionic and/or electronic transport along the aligned particle chains.

Polymer/ceramic composite materials are of great interest for their many potential applications because of their ability to combine at least two properties of the constitutive elements: particles and matrix. In most cases, such enhanced properties are required only in one direction. Orthotropic materials can be elaborated by applying an ac electric field to form particle chain structures in the direction of the electric field due to the dielectrophoretic interactions affecting the particles. However, there is still a lack in the understanding of the impact of the structures on the properties of the material. The aim of this study is to propose a predictive model for the evolution of the permittivity during the chain formation, by including micro- and macroscopic phenomena. The chaining model is based on dipole–dipole interactions and the dielectric permittivity is computed through a finite element method. In parallel, an experimental study is performed with online permittivity measurements of composites during chaining. The developed model is able to predict the experimental results from 1 vol% while taking into account parameters such as the resin viscosity and permittivity and the transient evolution of the applied electric field. The formation of particle chains inside a material has applications in many domains such as electrorheological fluids, anisotropic composites, self-recovery materials etc. Such a developed model is a valuable tool for the tailoring of materials.

International audience; As 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.

International audience; New wide-band gap semiconductors (SC) for power electronics such as SiC, GaN and diamond will allow higher power densities, leading tohigher operating temperatures. However, the surrounding materials will also undergo an increase in temperature, meaning that a parallel effort is needed in SC packaging technologies research. One of the essential components, the substrate, is used to insulate electrically the SC from the rest of the system, drain the generated heat and provide a path to connect the SC to the rest of the system. Direct bonded copper (DBC) and active metal-brazed (AMB) substrates have limited temperature and cycling operation, owing to the large differences in the thermal expansion coefficients between the ceramics and the metals. In this work we propose a new and original substrate technology based on two co-sintered ceramics: an insulating ceramic (AlN) and a conductive one (TIN). The microstructure, the chemical compatibility and the electrical properties indicate that the proposed substrate could operate at a temperature above 200 1C the current substrate technologies, which makes it particularly attractive for high-temperature power electronics applications.

the objective of this work is to provide new information on correlation between mechanical and electrical properties of alumina (Al 2 O 3 ), particularly the understanding of the dielectric breakdown mechanism. We propose to study the impact of the mechanical prestress on the short-term dielectric strength of the ceramic. Flexural stress is applied to induce or propagate cracks within alumina plates of 635 µm. We assume that the mechanical prestress impacts the initial crack size in the material, hence diminishing the breakdown voltage according to the electromechanical breakdown model.

Two characterization techniques, the steady-state Conduction Current (CC) and the low frequency Dielectric Spectroscopy (DS) are reviewed and compared to each other in order to choose the most suitable method for evaluating the static electrical conductivity (σDC) of an insulating material. In the case of polymeric materials operating above 200°C, the DS appears as being better suited. These techniques are applied for insulating materials identified as good candidates for high temperature (HT) applications and new results are presented. HT polyimide, parylene films and silicon nitride substrates are studied. These examples highlight the σDC magnitudes for temperatures up to 400 °C, as well as other relevant parameters to be taken into account for practical applications.

Nano-sized Ba1−xLaxTiO3 (0.00 ≤ x ≤ 0.14) powders were prepared by a coprecipitation method and calcined at 850 °C in air. The corresponding ceramics were obtained by Spark Plasma Sintering (SPS) at 1050 °C. These ceramics are oxygen deficient and are marked as Ba1−xLaxTiO3−δ. Both powders and ceramics were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The effect of lanthanum concentration on the densification behavior, on the structure and the microstructure of the oxides was investigated. Average grain sizes are comprised between 54 (3) nm and 27 (2) nm for powders, and 330 (11) nm and 36 (1) nm for ceramics according to the La-doping level. Powders crystallize in the cubic (or pseudo-cubic) perovskite phase. The structure of ceramics consists in a mixture of cubic (or pseudo-cubic) and tetragonal perovskite type phases. As the lanthanum content increases, the tetragonality of the samples decreases, as well as the grain size.

Development of microelectronic devices is driven by a large demand for faster and smaller systems. In the near future, colossal permittivity in nanomaterials will play a key role in the advances of electronic devices. We report on “colossal” permittivity values achieved in dense ceramics displaying ultrafine grain size ranging from 70 nm to 300 nm. Relative permittivity values of ∼ 106 at 1 kHz (0.1 < tand < 0.7) were obtained for Ba0.95La0.05TiO3–x ceramics. The colossal effective permittivity is related to an interfacial polarization and is achieved in nanomaterials by the activation of a high number of carriers and their trapping at the interfaces. Polarization carriers involving Ti3+ polaron is proposed to be at the origin of the observed colossal permittivity. These results may have an important technological impact since these ceramics display ultrafine grain size opening a new route to the fabrication of very thin dielectric films.

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f = 1 kHz unusual interesting dielectric properties, a so called “colossal” permittivity value up to several 106 with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films.

International audience; This paper deals with the study of the dielectric properties of polymer/high-k materials choosing an epoxy resin as the matrix and barium titanate (BaTiO 3) or strontium titanate (SrTiO 3) as micro-fillers (1 μm in diameter). Different studies on epoxy/BaTiO 3 and relatively few ones on epoxy/SrTiO 3 composites showed the increase in the permittivity with an increase in the filler content (from 0 to 50 vol.%). Whereas several authors have already investigated the dielectric strength of epoxy/BaTiO 3 , little is known regarding epoxy/SrTiO 3. In this paper, a comparison between these two types of composites will be presented. The main interest of SrTiO 3 is that the dielectric strength of the bulk ceramic material is 7 times-higher (35 kV/mm) than that of BaTiO 3 even though its dielectric constant is 10 times-lower (around 200-300).

In this paper, we have investigated the impact of a crystallization performed through a 300°C thermal curing on the dielectric properties and thermal characteristics of parylene N (PA-N) and parylene D (PA-D). Particularly, the evolution of the permittivity and loss factor in temperature and frequency ranges from −150°C to 300°C, and from 10−1 Hz to 106 Hz respectively, and the DC and AC dielectric strength have been measured. The results show a strong impact of crystallization on PA-N while PA-D remains unaffected.

This paper deals with the gas pressure and temperature dependence of partial discharge inception voltage (PDIV) of a developed gas-insulated power module as a high temperature and high voltage package. Increasing the gas pressure and temperature in the developed power module increases and decreases PDIV, respectively. Furthermore, the result of decreasing PDIV with increasing temperature is discussed by taking PD light emission image and simulating electrical field around the contact between a springs electrode and Si 3 N 4 dielectric chip which simulates SiC-SBD.

Since the observation almost 15 years ago of the so-called “colossal” dielectric permittivity behavior in CaCu 3 Ti 4 O 12 (CCTO) ceramics, several works have been undertaken to understand its physical origin interfacial polarization being the most likelihood. In this paper, ( C-V ) measurements, commonly used on semiconducting materials have been used to characterize CCTO samples. Their results may be described by a head-to-tail double metal-insulating-semiconductor (MIS) structure. A comparison between experimental and numerical simulation results of such a structure shows a good agreement, whatever the frequency range. Furthermore, this model explains the non-symmetrical behavior of the electrical response of this material, a property still not taken into account by today’s commonly known models.

This paper focuses on the study of high-capacitance embedded capacitors for power electronics applications, manufactured by the screen-printing technique. Thick film ferroelectric ceramic was printed on Ag/Al2O3 substrates and dried at 120 °C, then a pre-treatment was applied by cold isostatic pressing (CIP) before sintering. A comparison of the physico-chemical and electrical properties of the non-CIP and CIP samples is made. The results show that the thick film samples pre-treated by CIP have higher permittivity and higher dielectric strength compared to the untreated ones. The capacitor samples with CIP pre-treatment also exhibit higher capacitances, lower parasitic equivalent series inductance and series resistance; all key intrinsic dielectric properties relevant for applications. We propose the use of this process for obtaining the embedded capacitors that allow for high-capacitance values for decoupling and filtering in power electronics applications.

International audience; Low profile soft ferrite films constitute a competitive solution for the integration of micro-inductors on silicon in low-power medium frequency DC-DC conversion applications. High resistivity of soft ferrites is indeed a major advantage for operating frequencies in the range of 5-10 MHz. We have studied several soft ferrites including commercial ferrite films and in-house made ferrites. Test inductors were fabricated at wafer level using micromachining and assembling techniques. The proposed process is based on a sintered ferrite core placed in between thick electroplated copper windings. The low profile ferrite cores of 1.2 × 2.6 × 0.1 mm3 were produced by two methods issued from green tape-casted films or ferrite powders. This article presents the magnetic characterization of the fabricated ferrite cores cut and printed in rectangular shape and sintered at different temperatures. The comparison is made in order to find out the best material for the core that can offer the micro-inductor a high inductance in the range of 200-1000 nH at 6 MHz and that generate the smallest losses. With commercial ferrite core, it is demonstrated, thanks to a test inductor, that an inductance density of 215 nH.mm-2 up to 6 MHz could be reached. Extracted losses at 6 MHz, under 10 mT are in the range of 0.7 to 2.5 W.cm-3.

The purpose of our work is to develop a power module using gas insulation and bonding-less interconnections to avoid the reliability issues known to current packaging technology. This paper deals with the temperature dependence of partial discharge inception voltage (PDIV) of a developed module up to 300 °C. In addition, an attempt is achieved to confirm half-wave rectifying operation up to 300 °C using SiCSchottky barrier diodes (SiC-SBD). From these results, the PDs were not detected up to 1500 V and 300 °C, proving its ability to be used in high temperature applications.

The present study aims to put to profit a newly developed insulating nano-alumina (Al2O3) hydrate coating technique applied to electrically conductive particles. This paper deals with the morphology of the nano-alumina hydrate coating and its impact on dielectric properties of the composite materials. For the base polymer, a bisphenol-A type epoxy resin and an alicyclic polyamine hardener were used. The following four kinds of micro particles were used as conducting or semi-conducting core materials; Aluminum, Graphite, Zinc, and Silicon carbide. The particles were coated by a nano-alumina hydrate using a hydrothermal synthesis method. Alumina micro particle was also used as a reference. Dielectric properties were characterized by broadband dielectric spectroscopy (10 -2 -10 6 Hz) and volume resistivity measurements.

Since their discovery, 13 years ago there has been a lot of interest in the colossal dielectric permittivity ceramic materials. These materials derived from or are related to the calcium-copper titanate (CaCu 3 Ti 4 O 12 ) exhibit large values of permittivities (e r > 10 5 ), well above those obtained with ferroelectric ceramics. Along the large values of permittivity, good thermal stability is exhibited over a large temperature range (-100 to 200°C) making this family of materials a potential candidate for microelectronics and power electronics integration. The discussion over the physical origin of the large dielectric constants tends, by a large consensus, to describe the permittivity as the result of the formation of an Internal Barrier Layer Capacitance (IBLC). This would mean that internal barriers control the full electrical characteristics of the ceramics. Nevertheless, despite large attempts to control and diminish the dielectric and conduction losses of these materials, the whole electrical characteristics do not allow them yet to jump from fundamental research to real applications. In this paper, we present a state of the art of the colossal dielectric constant materials, providing a review of the different mechanisms that aim to explain the large values of permittivity. An analysis of the different proposed mechanisms and their limitations will be discussed. We will also present recent developments made by our group on the characterization of these ceramic materials that bring light into new characterization and analysis strategies to describe these materials and may, in some ways, explain why they are still not in use in today's highly demanding applications.

Aluminium oxide (Al2O3), Aluminium Nitride (AlN) and Silicon Nitride (Si3N4) are ceramics commonly used in power electronics modules, and are the current candidates for high temperature applications. A first study has previously shown different charge displacement behaviors at high temperature (up to 400°C). Surface potential measurements revealed a fast decay for AlN and Al2O3 with the increase of the temperature while on Si3N4 charges remain on surface at 400°C. However, these charges may move through the surface or be injected in the volume of the ceramics. - Surface potential results are correlated with broadband impedance spectroscopy and to current-voltage measurements of the ceramics. AlN and Al2O3 have a resistive-like behavior at high temperature, characterized by a space charge limited conduction current mechanism at high fields. For Si3N4, despite its resistivity decreases with temperature, it shows a dielectric behavior and an ohmic conduction mechanism over the studied temperature and field ranges.

International audience; Ferrite-based micro-inductors integrated on silicon are proposed as potential hybrid integration in low-power medium frequency DC-DC power converters. The inductors are fabricated at wafer level using micromachining and flip chip assembling techniques. The proposed process is based on a sintered ferrite core placed in between thick electroplated copper windings on two silicon substrates. The low profile (110 μm thick) ferrite cores are produced by thermal bonding of two green tape-casted films of 1.2 × 2.6 × 0.2 mm3, then micro-milled to the final dimensions before sintering. This paper presents the magnetic characterization of the sintered ferrite films cut in rectangular shape and sintered at different temperatures. It is concluded that these small dimension cores could be used in the frequency range applications of 5-10 MHz and would potentially allow a high inductance density in a 3D integration process.

This paper deals with I-V characteristics of SiC-SBD and rectifying capabilities of four SiC-SBD using a new package under the temperature up to 300°C. As regards of the I-V characteristics, as the temperature increases, the threshold voltage decreases and the reverse current increases dramatically. This increase of the reverse current of the diodes under study leads to the limitation of the rectifying characteristics of the proposed packaging. This study demonstrates the ability of the later to be used in a harsh environment ones the components problems will have been solved.

International audience; 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.

Perlite is an igneous mineral composed by silicon, aluminum, oxygen and water. It can be expanded by heating it up at temperatures above 870 °C, then it becomes plastic, and the steam formed inside pressures out of the mineral. Maximum expansion is possible if the particles are heated up quickly, since the expansion degree strongly depends on the remaining water in the particles at the time that they reach the temperature where they become plastic. The typical expansion process consist in pouring the particles in rotary kilns operated with natural gas, but it is proposed in this research that the particles can be heated quickly with microwaves at 2.45 GHz. Particles of 0.08 cm and 0.018 cm of average diameter were expanded 10 to 20 times.

In this paper the properties of epoxy-based nanocomposites with multilayer graphene (MLG) as a filler are investigated. In particular dielectric properties, such as conductivity and polarization characteristics, are studied as a function of MLG filler content, ranging between 0.05 wt.% to 0.5 wt.%, through charging / discharging current measurements at the field of 25 kV/mm and two temperatures (20°C and 60°C). Experimental results show that steady-state value of the charging current (conduction current) increases remarkably with a higher filler content. Moreover, Fourier analysis of discharging currents shows that dielectric susceptibility at very low frequency is significantly affected by the addition of MLG nanoplatelets. These results indicate that even a small percentage of MLG nanofiller is able to modify the electrical properties of the final nanocomposite.

International audience; Graphene is a carbon-based allotrope material which, since very few years, appears as very exciting due to incredible physical properties such as a high electron mobility (250,000 cm 2 /Vs), a high thermal conductivity (5,000 W/mK) and a high Young's modulus (1 TPa). Obtained from the exfoliation of graphite pristine in the form of multilayer graphene (MLG) nanoplatelets, it focuses an increasing attention when mixed to a polymer matrix to produce MLG/polymer nanocomposites. Up to now, few studies have mainly reported on the effects on the thermal conductivity of various MLG/polymer nanocomposites. However, no study has been led on their dielectric properties. In this paper, we propose to study the impact of MLG on the dielectric permittivity and losses of an epoxy nanocomposite with different low nanoflakes filler contents from 0.005 to 0.5 wt.%, chosen below the electrical percolation threshold. The study will be performed both in wide temperature and frequency ranges in order to highlight the influence of the MLG nanoplatelets on the epoxy relaxation dynamics using a broadband dielectric spectrometer from Novocontrol. The MLG loading causes an increase of the polarizability of the composite material and thus the dielectric losses. At high temperature, at small MLG additions, the graphene particles interfere with the cooperative movements associated with the vitreous transition temperature, increasing the permittivity and decreasing the losses.

Using wideband gap semiconductors (SiC, GaN) appears as possible solution to the growing demand for the development of high temperature, high frequencies power electronics. This will help downsizing the current Power Electronics systems and extend their range of operation conditions. The aim of this paper is to present the results obtained concerning the packaging of a full-wave rectifying bridge using SiC Schottky Barrier Diodes, capable to operate above 300°C. The diodes are rated 1200 V and 50 A. The proposed packaging is based on an insulating-gas encapsulation and stacking of the components. The rectifying function was tested by feeding a current trough the bridge at room temperature, and while applying high voltage in an insulating gas-filled heated chamber. Results show that the type of interconnection proposed can withstand at least 10 A and 200 V in rectifying conditions. Gas encapsulation allowed for an operation of the diodes under high voltage conditions up to 350°C even if at this temperature diode leakage current is too high to perform an appropriate rectification.

International audience; A polymer nanocomposite (P-NC) has been manufactured using different processes based on ultrasonication (US) and particles surface treatment. The moisture absorption was detected with different characterization method such as Fourier transform infrared spectroscopy, isothermal thermogravimetric analysis and electrical characterizations. The use of particle surface treatment allows decreasing O-H absorption into P-NC. Moreover, an efficient US process has been found to improve the P-NC hydrophobicity. The desorption kinetics of moisture has also been studied for different P-NC preparation processes, using isothermal characterizations at 150 degrees C. By controlling the chemical surface state of the nanofillers and the dispersion, it was observed a good stability of the conduction current density, in the same order of that of the base polymer.

The aim of this paper is to determine the influence of the temperature on the surface potential decays after corona charging in a needle-to-plane arrangement for different ceramic materials. The materials under study are generally used as substrates in Power Electronics modules and their behaviours in the high temperature range are insufficiently known. A comparison is made between Al 2 O 3 , Si 3 N 4 and AlN. A correlation is proposed between the surface potential change over time and their broadband dielectric response.

This paper deals with the filler movement and the dielectric properties in epoxy composites during curing process under a uniform electric field. For the base epoxy resin, a bisphenol A type epoxy resin and an alicyclic polyamine hardener were used. A plate-like micro-size Al 2 O 3 powder was used as filler. After fully mixing the base resin, fillers, and hardener, they were vacuum-defoamed, poured into a mold having two parallel electrodes, and subsequently heat-hardened. In-situ optical observation and continuous measurements of viscosity and impedance were performed. Broadband dielectric spectroscopy was also carried out after full hardening process. As a result, the formation of chain-like structures of particles is related to both the magnitude of the electric field and its frequency. The dielectric properties of the fluid also impact the formation of chain-like structures.

The keys to successful high power electronic systems are located as much in the ability to build high temperature power devices and to package them with the appropriate materials, as in the aptitude to reduce and control switching and conduction power losses. Particularly, high temperature low loss operation allows an increase in the power rating of these devices. The recent development of wide band gap semiconductor devices should allow improving power electronic systems. Wide band gap semiconductor materials, especially the most mature silicon carbide (SiC), should allow the electronics operation at high junction temperatures (>200°C), high voltages (>10 kV) or in harsh thermal environment, with faster switching and lower power losses active devices than the silicon (Si) counterparts. Such SiC devices impose more severe electrical and thermal stresses to the surrounding insulating materials (polymeric passivation and encapsulation materials and ceramic substrates). Lots of improvements have already been built-up at the die level; however, superior device performance degrees could be reached using higher performance insulation materials. Among the power device packaging materials for a high temperature operation, typical organic passivation and encapsulation appear nowadays as the most sensitive to the thermal constraints (Tmax=250 °C). Moreover, even if ceramic materials present a high isothermal stability (up to 600°C) they are very sensitive to the large passive or active thermal cycling induced by the power devices or by severe environmental constraints during operation. Therefore, research on high temperature dielectric materials tries to identify new polymeric and ceramic materials electrically, thermally and mechanically suited for the packaging of SiC power devices and to determine their effective limits (properties and durability). In this chapter after a section on the high temperature applicative needs and the new thermal and electrical constraints imposed by SiC devices on the surrounding insulating materials, a complete review of the polymers and ceramics insulating materials which are reported to potentially answer to the packaging issues is carried out through a presentation of their different main physical properties and the sensitive aging parameters in link with microstructure. Among the polymeric materials, BPDA/PDA polyimide (PI), fluorinated parylene (PA-F), polyamide-imide (PAI), and silicone (PDMS) will be studied. On the other hand, mainly aluminium nitride (AlN) and silicon nitride (Si3N4) ceramics will be presented.

Two colossal dielectric permittivity (CDC) materials, CaCu3Ti4O12 (CCTO) issued from conventional sintering with grain sizes between 20 and 30 µm and SPS sintered La0.05Ba0.95TiO3−δ (BTL-SPS) with grain sizes between 50 and 100 nm, are characterized by simple electrical measurements (Sawyer–Tower and I(V)). Whatever the type of measurements performed, the results depend, on the one hand, on the relative position of the sample in the measuring setup and, on the other hand, on the type of surface treatment achieved on the sample. A clear demonstration of the non-isotropic character of the materials under study is achieved. The non-symmetrical electrical response observed in these two different materials seems to be independent of microstructure and composition, and could be related to the overall phenomena at the origin of the colossal values of permittivity.