Hydrothermal synthesis of nanosized BaTiO3 powders and dielectric properties of corresponding ceramics

BaTiO3 fine powders were synthesized by hydrothermal method at 150 °C or 250 °C for 7 h, starting from a mixture of TiCl3 + BaCl2 or TiO2 + BaCl2. The size of the crystallites is close to 20 nm whatever the starting mixture and the reaction temperature. These powders are well crystallized and constituted of a mixture of the metastable cubic and stable tetragonal phases. The ceramics obtained after uniaxial pressing and sintering at 1250 °C for 10 h or 20 h present high densification (up to 99.8%). The Curie temperature (Tc) and the electrical permittivity ( r) of the ceramics strongly depend on the type of titanium source that has been used for preparing the powder and on the sintering dwell time. Particularly, Tc is shifted towards lower temperature when TiCl3 is used. The permittivity value at Tc of BaTiO3 sintered at 1250 °C for 10 h reaches 7000 and 11,000 with respectively TiCl3 and TiO2 used as titanium source

Influence of pulse current during Spark Plasma Sintering evidenced on reactive alumina–hematite powders

Spark Plasma Sintering (SPS) is increasingly used. The temperature and current are not independent parameters, making it difficult to separate the current intrinsic role from Joule heating. There is a debate on whether there are any specific SPS mechanisms. The influence of a key parameter, the (on:off) pulse pattern, is studied on the SPS of reactive α-Al2−2xFe2xO3 (x = 0.02; 0.05; 0.07; 0.10) powders. Changing it modifies the current crest intensity and has a great influence on the materials microstructure. Comparisons with runs where the current is blocked and hot-pressing reveal three competing phenomena: formation of FeAl2O4, dominant in the core and not peculiar to SPS, formation of Fe, producing Fe-Al2O3 composite surface layers, and most notably electrical-field induced diffusion of Fe3+ ions towards the cathode, which could have far-ranging implications for the consolidation of ionic materials and the in situ reactive shaping of composites and multimaterials

Highly integrated power electronic converters using active devices embedded in printed-circuit board

International audienceIn this paper, we present a short overview of the power dies interconnects based on PCB technology, then we focus on embedding of power dies in the PCB material. In the second part of the article, we present in details the technology we developed to embed dies in PCB. Results show that the electrical performance of the die remains satisfying after embedding, but that dies with a copper topside metal layer are required for this process

Electro-thermal measurements and finite element method simulations of a spark plasma sintering device

Current, voltage and temperature measurements were performed at different points of the system to identify the controlling parameters of the spark plasma sintering (SPS) process. The very low inductance effects despite the high intensity current circulating through the SPS column justifies the use of Joule heating to characterize the phenomenon. The measurements also enabled the improvement and validation of an earlier electro-thermal numerical model developed using the finite element method (FEM). It has been shown that the electrical resistivity and the thermal conductivity of each of the elements are crucial parameters for the simulations. These parameters strongly modify the current modeled, thereby affecting the temperature distribution throughout the SPS column

Pulse analysis and electric contact measurements in spark plasma sintering

In order to model the current density distribution and the temperature changes of the tools used during a spark-plasma-sintering (SPS) cycle, the variation of the power delivered by an SPS machine and the graphite-Papyex®-graphite electrical contacts were studied experimentally. The electric device was also characterized; in particular current pulse characteristics and their behavior with time were studied in various conditions of temperature, pulses sequences, materials and total electric power dissipated. It is well known that the performance of an electric contact is dependent on the applied pressure and the temperature. First, by varying the pressure during the SPS cycle the effect of the electric contacts is clearly seen. Secondly, in order to determine the behavior of such contacts experimentally over a pressure range of 10–50 MPa and temperatures of 50–800 °C, a Dœhlert experimental design was used

Synthesis and electrical characterization of monocrystalline nickel nanorods and Ni-CNT composites

Aerospace vessels require electrically conductive, light weight frames to minimize damage from electromagnetic radiation, electrostatic discharge and lightning strikes while economizing fuel. Nickel nanowires and hybrid nickel-carbon nanotube materials are suitable nanostructures to ensure high conductivity at low mass loading. Monocrystalline nickel structures have even better conduction properties than the polycrystalline equivalent due to possessing less particle-particle junctions. We have developed a solutionbased method that produces monocrystalline nickel nanowires via the decomposition of metalorganic precursors in the presence of self-assembled surfactants. The resulting wires are approximately 20 nm wide by 1.5 µm in length. These wires have a morphology consisting of semi-flattened rods with pyramidal ends. Despite the changing dimensions between the nanorod body and its head, there was no disruption in the crystallographic orientation, as observed with HRTEM and diffraction patterns. The nickel nanostructures were exposed to air for several weeks, but no oxidation was detectable by magnetic measurement, i.e. the saturation magnetization corresponds to Ni0 and no bias is observed in the hysteresis loops. It seems that the long alkyl chain amine surfactant, in addition to being a structuration agent, remains at the surface of the Ni wires after washing and acts as a protective layer. The magnetic field around Ni nanowires was imaged using electron holography. Each Ni wire is a magnetic monodomain. Routes to prepare hybrid nickel-CNT materials were explored using chemical vapor deposition in a fluidized bed, solution chemistry and dry preparation in a Fisher-Porter reactor. Different nickel compositions and material morphologies resulted, depending on the preparation technique. The nickel nanorods and hybrid materials were incorporated into carbon fiber-reinforced polymer composites. The electrical conductivity as a function of wt% loading was measured, showing promise for these materials in discharging electrostatic charges

Architecture of the fungal nuclear pore inner ring complex

The nuclear pore complex (NPC) constitutes the sole gateway for bidirectional nucleocytoplasmic transport. We present the reconstitution and interdisciplinary analyses of the ~425-kDa inner ring complex (IRC), which forms the central transport channel and diffusion barrier of the NPC, revealing its interaction network and equimolar stoichiometry. The Nsp1•Nup49•Nup57 channel nucleoporin hetero-trimer (CNT) attaches to the IRC solely through the adaptor nucleoporin Nic96. The CNT•Nic96 structure reveals that Nic96 functions as an assembly sensor that recognizes the three dimensional architecture of the CNT, thereby mediating the incorporation of a defined CNT state into the NPC. We propose that the IRC adopts a relatively rigid scaffold that recruits the CNT to primarily form the diffusion barrier of the NPC, rather than enabling channel dilation

Etude de l'intégration des protections en électronique de puissance (utilisation des varistances à base d'oxyde de zinc)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Etude et mise en oeuvre de techniques d'assemblages hybrides pour l'intégration tridimensionnelle en électronique de puissance

L'amélioration de l'efficacité énergétique du vecteur électricité passe par une conception très intégrée des systèmes électroniques, notamment des convertisseurs d'énergie électrique. La tendance générale est à une forte intégration des fonctionnalités d'une part et à l'augmentation de la densité de puissance d'autre part. C'est particulièrement le cas des systèmes nomades et des systèmes intervenant dans les transports (automobile, ferroviaire et aérien), avec en plus, l'exigence d'une très haute fiabilité. Ces applications nécessitent de nouvelles architectures matérielles d'intégration avec des contraintes toujours plus importantes vis-à-vis de la réduction du volume, du poids et des coûts tout en maintenant un niveau de fiabilité élevé. De plus, les environnements ont tendance à être plus sévères (plus chauds et quelquefois plus froids). À ce titre, l'électronique de puissance "haute température" est un enjeu majeur pour le futur. Une des pistes les plus prometteuses pour l'augmentation de la compacité des systèmes est l'intégration tridimensionnelle. En particulier, le passage des modules de puissance " planar " actuels (où toutes les puces sont coplanaires, et refroidies uniquement par leur face arrière) à des structures 3D permettra d'améliorer simultanément les performances thermiques (refroidissement double face), électriques (réduction des inductances et résistances parasites) et électromagnétiques. C'est dans ce contexte que les technologies d'interconnexion et d'assemblage dans les modules de puissance jouent un rôle fondamental. Grâce aux révolutions des nano technologies, un mode d'assemblage hybride a été proposé sous le nom " nano scratch " consistant en la mise en compression de deux surfaces dont l'une au moins porte une structure de nano fils, et de les faire s'interpénétrer l'une dans l'autre, afin de réaliser un assemblage mécanique mais aussi conducteur électrique et thermique. Ce type d'assemblage semble être un candidat prometteur pour remplacer la technique de brasure classique. Il permet d'une part de favoriser l'intégration 3D, et d'autre part, de rendre le procédé plus économique à l'aide de sa réalisation à froid. La contribution principale de ce travail de thèse se situe essentiellement en l'étude et la mise en oeuvre d'un procédé d'assemblage de type " nano scratch " en utilisant des nano fils de cuivre réalisés par la voie du dépôt électrochimique en vue de l'appliquer dans le domaine électronique de puissance. La première partie du mémoire est consacrée à une synthèse des différents constituants d'un module de puissance...In Power Electronics Applications, the general trend is to develop integration leading to an increase of the power density and a decrease of volume, weight and cost while maintaining a high level of reliability. It is particularly, the case for mobile systems or systems involved in transportation (automotive, railways and aeronautics). One of the most promising issues to improve the compactness of the systems is three-dimensional (3D) integration. The shift from classic power modules (where all the chips are coplanar, and the heat only transferred by the back side) to 3D structures allows improving simultaneously the thermal performances (double sides' cooling), the electric ones (reduction of parasitic inductances and decrease of the resistances) and the electromagnetic ones. Interconnections and assembly, improperly called "packaging", play a fundamental role in this field. In this Thesis, thanks to development of nanotechnologies, a hybrid assembly method is proposed. Named " nano scratch ", it consists in a structure of nano copper wires, deposited on at least one of the surfaces to be joined. The interpenetration of these nanowires is then achieved by pressing the surfaces against each other. Hence, not only a mechanical assembly but also an electrical and a thermal connection is realized. This type of assembly is a promising candidate for the replacements of solder technology. This simple and low cost process may favor 3D integration in power electronics applications. In a first part, the different components of a power module are reminded, Then, a state of art of the different interconnection technologies and assemblies used in power modules is presented. The second chapter focuses on an evaluation of the different methods used to fabricate the nanostructures. Electroplating through a nano porous membrane appears to be the most reproducible one and will be the one studied in this thesis. The last chapter, the principle and implementation of an electro-thermo mechanical assembly are presented and tested...TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Etat de l’art de l’intégration en électronique de puissance et axes de recherches associés

National audienc