Zarel Valdez-Nava

Dielectrics for High Temperature SiC Device Insulation: Review of New Polymeric and Ceramic Materials

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.

Fundamental investigation of dielectric phenomena in epoxy composites during curing process under a uniform electric field

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 Al2O3 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.

Dynamics of particle chain formation in a liquid polymer under ac electric field: modeling and experiments

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.

Packaging of SiC-SBD for high temperature operation

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.

Non-symmetrical electric response in CaCu3Ti4O12 and La0.05Ba0.95TiO3−δ-SPS materials

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.

Effects of filler content on dielectric properties of epoxy/SrTiO3 and epoxy/BaTiO3 composites

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 (BaTiO₃) or strontium titanate (SrTiO₃) as micro-fillers (1 μm in diameter). Different studies on epoxy/BaTiO₃ and relatively few ones on epoxy/SrTiO₃ 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/BaTiO₃, little is known regarding epoxy/SrTiO₃. In this paper, a comparison between these two types of composites will be presented. The main interest of SrTiO₃ is that the dielectric strength of the bulk ceramic material is 7 times-higher (35 kV/mm) than that of BaTiO₃ even though its dielectric constant is 10 times-lower (around 200-300).

Space charge accumulation on multilayer graphene/epoxy nanocomposites

Nanocomposite materials are nowadays widely studied. It is often desired, by the addition of nanoparticles in an insulating matrix, to improve electrical and thermal properties. However, few types of nanoparticles enable improvement of combined electrical and thermal properties. Here the influence of the presence of graphene nanoparticles in an epoxy matrix on space charge accumulation is studied. Thus, several percentages of graphene nanoparticles (0.05 wt.%, 0.1 wt.%, 0.5 wt.%), voluntarily chosen below the electrical percolation threshold and an epoxy resin as reference, are studied in order to evaluate the influence of the content of the nanoparticles in the epoxy matrix on space charge accumulation. Space charge measurements have been performed using the Thermal Step Method. The influence of the applied dc electric field and temperature during the poling has also been studied.

Broadband dielectric spectroscopy of multilayer graphene/epoxy nanocomposites

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 cm2/Vs), a high thermal conductivity (5,000 W/mK) and a high Youngs 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.

Soft ferrite cores characterization for integrated micro-inductors

Ferrite-based micro-inductors are proposed for hybrid integration on silicon for low-power medium frequency DC-DC converters. Due to their small coercive field and their high resistivity, soft ferrites are good candidates for a magnetic core working at moderate frequencies in the range of 5?10 MHz. We have studied several soft ferrites including commercial ferrite film and U70 and U200 homemade ferrites. The inductors are 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 ? 0.2 mm3 are produced by two methods from green tape-casted films and ferrite powder. This paper presents the magnetic characterization of the sintered ferrite films 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 reach the required inductance (470 nH at 6 MHz) under 0.6A current DC bias and that generate the smallest losses. An inductance density of 285 nH/ mm2 up to 6 MHz was obtained for ESL 40011 cores that is much higher than the previously reported devices. The small size of our devices is also a prominent point.

Online optical and dielectric monitoring of anisotropic epoxy/BaTiO3 composite formation tailored by dielectrophoresis

Anisotropic composites can be obtained by applying an alternating (ac) electric field, forming particle chains in its direction. The control of the particle chains will directly impact the final properties of the composite. Nevertheless, up to now the monitoring of the particle chain formation has only been made by direct optical or post-curing observations. A new technique for the monitoring of the particle dielectrophoretic alignment is proposed, based on the online measurement of the dielectric permittivity. Epoxy/barium titanate (BaTiO3) composites, in the range of 0.25 vol% to 20 vol% of BaTiO3 microparticles, are cured while an ac field (600 Vrms mm−1) is applied. The ac current magnitude and the phase shift angle are measured to determine the dielectric properties of the composite. The same experiment is achieved under optical microscope observation for 0.25 vol% to correlate the changes of the composite dielectric properties to the particle chain formation. As a result, the permittivity variations can be correlated to the particle chains formation and to their growth.