Microstructure of Ba1−xLaxTiO3−δ ceramics sintered by spark plasma sintering

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

Influencia de la alúmina como absorbedor de microondas en la reacción de formación de espinel alúmina-magnesia

Tesis (Maestría en Ciencias de la Ingeniería Mecánica con Especialidad en Materiales) UANLUANLhttp://www.uanl.mx

Sinterización de manganitas NI-FE empleando microondas como fuente de energía

Tesis (Doctor en Ingeniería de Materiales) U.A.N.L.UANLhttp://www.uanl.mx

Structural characterization of dense reduced BaTiO3 and Ba0.95La0.05TiO3 nanoceramics showing colossal dielectric values

BaTiO3−x and Ba0.95La0.05TiO3−x nanoceramics showing colossal permittivity values have been characterized. While starting powders are of cubic symmetry, X-ray and Neutron Diffraction techniques and Raman Spectroscopy measurements show that the one-step processed ceramics obtained by Spark Plasma Sintering (SPS) contain cubic and tetragonal phases. Rather large oxygen deficiency determined in such ceramics by Electron Micro Probe analysis and Electron Energy Loss Spectroscopy analyzes is explained by the presence of Ti3+, as evidenced by X-ray Photoelectron Spectroscopy measurements. Transmission Electron Microscopy and High Resolution Transmission Electron Microscopy show that these ceramics contain 50–300 nm grains, which have single-domains, while grain boundaries are of nanometer scale. Colossal permittivity values measured in our dense nanoceramics are explained by a charge hopping mechanism and an interfacial polarization of a large number of polarons generated after sample reduction in SPS apparatus

Co-fired AlN–TiN assembly as a new substrate technology for high-temperature power electronics packaging

New wide-band gap semiconductors (SC) for power electronics such as SiC, GaN and diamond will allow higher power densities, leading to higher 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

HEMI-INATENCIÓN EN NIÑOS

Hemineglect is a neuropsychological syndrome observed in adults as a consequence of right hemisphere damage, affected patients tend to neglect objects located on the left side of the visual field. The present work reviews available data that have documented hemineglect in children. This syndrome occurs in children as a consequence of brain damage or as a maturational delay during development. Hemineglect produces specific type of learning disorders, which interfere with the acquisition of reading (dyslexia), writing (dysgraphia) or numeric calculus (dyscalculia). It is important to develop better detection, diagnosis, and treatment strategies of this syndrome in children.La hemi-inatención es un síndrome neuropsicológico que se presenta en adultos como resultado de un daño en el hemisferio derecho, el paciente con esta lesión tiende a ignorar los objetos que se encuentran en el lado izquierdo de su campo visual. El presente trabajo revisa los trabajos que documentan la forma en que se presenta este síndrome en niños. En niños puede ocurrir hemi-inatención a consecuencia de una lesión cerebral o debida a un retraso maduracional durante el desarrollo. La presen-cia de hemi-inatención en niños produce un tipo específico de trastorno de aprendizaje, que interfiere con la adquisición de la lectura (dislexia), la escritura (disgrafía) o el cálculo numérico (discalculia). Es muy importante estudiar este síndrome en niños, así como desarrollar mejores estrategias de detección, diagnóstico y tratamiento

Influence of Oxygen Substoichiometry on the Dielectric Properties of BaTiO3-δNanoceramics Obtained by Spark Plasma Sintering.

Oxygen-deficient BaTiO3-d nanoceramics were prepared by spark plasma sintering. Partially reduced raw nanopowders led to unusual dielectric properties. A short postsintering treatment was performed to reach a high er/tan d, which makes them attractive for industrial applications such as low-frequency capacitors. Surprisingly, our ceramics also remained black even after annealing for 5 days at 850°C in air, indicating the presence of barriers against oxygen diffusion. This exceptional behavior in pure barium titanate was consistent with a core–shell structure made of semiconductive grains and insulating grain boundaries, due to the presence of Ti3+/Ti4+ and oxygen vacancies

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

Un nuevo hospedero y distribución geográfica de Optatus palmaris Pascoe 1889 (Coleoptera: Curculionidae) en México.

The diurnal weevil Optatus palmaris Pascoe was found on Annona muricata L. (Anonaceae) fruits in Nayarit, Mexico which represents a new plant host record and extends the weevil distribution. Other plant hosts and damages produced are mentioned.