Sensitivity of SnO2 non-ohmic behavior to the sintering process and to the addition of La2O3

The work reported here consisted of a study of the sensitivity of the nonlinear electrical properties of dense SnO2. CoO ceramic systems to low concentrations of La2O3, sintering temperature and cooling rates. The nonlinear electrical properties of these systems were found to increase with decreasing cooling rates, a behavior attributed to the CoO solid state reactions at temperatures below 1000 degreesC. Post-annealing treatment in N-2-rich atmospheres strongly decreases the non-ohmic behavior of SnO2. CoO ceramic systems. However, this behavior may be restored through thermal treatment in an O-2-rich atmosphere. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

SnO2, ZnO and related polycrystalline compound semiconductors: An overview and review on the voltage-dependent resistance (non-ohmic) feature

The present review describes mainly the history of SnO2-based voltage-dependent resistors, discusses the main characteristics of these polycrystalline semiconductor systems and includes a direct comparison with traditional ZnO-based voltage-dependent resistor systems to establish the differences and similarities, giving details of the basic physical principles involved with the non-ohmic properties in both polycrystalline systems. As an overview, the text also undertakes the main difficulties involved in processing SnO2- and ZnO-based non-ohmic systems, with an evaluation of the contribution of the dopants to the electronic properties and to the final microstructure and consequently to the system's non-ohmic behavior. However, since there are at least two review texts regarding ZnO-based systems [Levinson, L. M., and Philipp, H. R. Ceramic Bulletin 1985;64:639; Clarke, D. R. Journal of American Ceramic Society 1999;82:485], the main focus of the present text is dedicated to the SnO2-based varistor systems, although the basic physical principles described in the text are universally useful in the context of dense polycrystalline devices. However, the readers must be careful of how the microstructure heterogeneity and grain-boundary chemistry are capable to interfere in the global electrical response for particular systems. New perspectives for applications, commercialization and degradation studies involving SnO2-based polycrystalline non-ohmic systems are also outlined, including recent technological developments. Finally, at the end of this review a brief section is particularly dedicated to the presentation and discussions about others emerging non-ohmic polycrystalline ceramic devices (particularly based on perovskite ceramics) which must be deeply studied in the years to come, specially because some of these systems present combined high dielectric and non-ohmic properties. From both scientific and technological point of view these perovskite systems are quite interesting. (c) 2007 Elsevier Ltd. All rights reserved.

The dielectric suppress and the control of semiconductor non-Ohmic feature of CaCu3Ti4O12 by means of tin doping

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effect of polymeric matrix on morphology and photoelectric properties of polymer-quantum dots nanocomposites

Recently, CdTe semiconductor quantum dots (QDs) have attracted great interest due to their unique properties [1]. Their dispersion into polymeric matrices would be very for several optoelectronics applications. Despite its importance, there has been relatively little work done on charge transport in the QD polymeric films [2], which is mainly affected by their structural and morphological properties. In the present work, polymer-quantum dot nanocomposites films based on optically transparent polymers in the visible spectral range and CdTe QDs with controlled particle size and emission wavelength, were prepared via solvent casting. Photoluminescent (PL) measurements indicate different emission intensity of the nanocomposites. A blue shift of the emission peak compared to that of QDs in solution occurred, which is attributed to the QDs environment changes. The morphological and structural properties of the CdTe nanocomposites were evaluated. Since better QDs dispersion was achieved, PMMA seemed to be the most promising matrix. Electrical properties measurements indicate an ohmic behavior.

Réduction de la durée de vie des porteurs de charge dans le silicium noir par implantation ionique

Le but de ce projet est d’étudier l’effet des défauts cristallins sur les propriétés optoélectroniques de photodétecteurs fabriqué à partir de « silicium noir », c’est-à-dire du silicium dopé et microstructuré par impulsions laser femtoseconde, ce qui lui donne une apparence noire mate caractéristique. Des échantillons de silicium noir ont été recuits puis implantés avec des ions ayant une énergie de 300 keV (Si+), 1500 keV (Si+) ou 2000 keV (H+). Trois fluences pour chaque énergie d’implantation ont été utilisées (1E11, 1E12, ou 1E13 ions/cm2) ce qui modifie le matériau en ajoutant des défauts cristallins à des profondeurs et concentrations variées. Neuf photodétecteurs ont été réalisés à partir de ces échantillons implantés, en plus d’un détecteur-contrôle (non-implanté). La courbe de courant-tension, la sensibilité spectrale et la réponse en fréquence ont été mesurées pour chaque détecteur afin de les comparer. Les détecteurs ont une relation de courant-tension presque ohmique, mais ceux implantés à plus haute fluence montrent une meilleure rectification. Les implantations ont eu pour effet, en général, d’augmenter la sensibilité des détecteurs. Par exemple, l’efficacité quantique externe passe de (0,069±0,001) % à 900 nm pour le détecteur-contrôle à (26,0±0,5) % pour le détecteur ayant reçu une fluence de 1E12 cm-2 d’ions de silicium de 1500 keV. Avec une tension appliquée de -0,50 V, la sensibilité est améliorée et certains détecteurs montrent un facteur de gain de photocourant supérieur à l’unité, ce qui implique un mécanisme de multiplication (avalanche ou photoconductivité). De même, la fréquence de coupure a été augmentée par l’implantation. Une technique purement optique a été mise à l’essai pour mesurer sans contacts la durée de vie effective des porteurs, dans le but d’observer une réduction de la durée de vie causée par les défauts. Utilisant le principe de la réflexion photo-induite résolue en fréquence, le montage n’a pas réuni toutes les conditions expérimentales nécessaires à la détection du signal.

Multi-functional properties of CaCu3Ti4O12 thin films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Varistores à base de SnO2: estado da arte e perspectivas

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

SnO 2 ceramics with low electrical resistivity obtained by microwave sintering

The main aim of this study was to develop dense and conducting SnO 2 ceramics without precipitated phases on the grain boundaries, which was verified using field emission scanning microscopy (FE-SEM) coupled with an energy-dispersive X-ray spectroscopy (FE-SEM/EDS). Two sample groups were investigated, where the first sample group was doped with zinc while the second one was doped with cobalt. The ceramics were prepared using the oxides mixture method and the sintering was carried out in a conventional muffle oven as well as in microwave oven. The results obtained were found to be similar regarding the relative density for the two sintering methods while time and temperature gains were observed for the microwave sintering method. The relative densities obtained were nearly 95%, for the two sintering methods. Concerning the electrical characterization measurements-electric field x current density as well as the environment temperature, the ceramics obtained through the conventional sintering method presented non-ohmic behavior. For the microwave sintered ceramics, we observed an ohmic behavior with electrical resistivity of 1.3 Ωcm for the samples doped with ZnO/Nb 2O 5 and 2.5 Ωcm for that of the samples doped with CoO/Nb 2O 5. The FE-SEM/EDS results for the microwave sintered ceramics indicated a structure with a reduced number of pores and other phases segregated at the grain boundaries, which leads to a better conductive ceramic than the conventional oven sintered samples. The dilatometry analysis determined the muffle sintering temperature and the difference between the densification of cobalt and zinc oxides. The addition of niobium oxide resulted in the decrease in resistivity, which thus led us to conclude that it is possible to obtain dense ceramics with low electrical resistivity based on SnO 2 using commercial oxides by the oxides mixture technique and the microwave oven sintering method. Copyright © 2011 American Scientific Publishers All rights reserved.

Uma contribuição para o estudo do efeito causado pela névoa salina em isoladores cerâmicos de chaves fusíveis

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Influência dos defeitos cristalinos nas propriedades dielétricas e não-ôhmicas do 'CA' 'CU IND.3' 'TI IND.4' 'O IND.12'(CCTO)

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Propriedades multifuncionais do CaCu3Ti4O12: estudo dos mecanismos e suas aplicações

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Estudo, construção e caracterização de diodos orgânicos tipo Schttky

During the twentieth century the inorganic electronics was largely developed being present in various industrial equipment or household use. However, at the end of that century were verified electronic properties in organic compounds, giving rise to the field of organic electronics. Since then, the physical properties of elementary devices such as diodes and organic transistors have been studied. In this work was studied the properties of diode devices fabricated with a semiconductor polymer, the poly-o-methoxyaniline (POMA). Devices containing electrodes of Au and Al were fabricated with semiconductor polymer of different doping levels. We found that the rectifying behavior for the heterojunctions metal/polimer are reached only for high doping level (with conductivity greater than 1,77. 10-9 S / cm), which gives the devices characteristic of a Schottky diode. The rectifying behavior was observed for electric fields of low magnitude, below the operating field (~ 600 V/cm), while for electric field greater than 600 V/cm the a linear behavior I vs.V was obtained. We determined that this Ohmic behavior arises from the charge transport over the volume of the semiconductor material after the lowering of the metal/semiconductor barrier. In devices with weakly doped semiconductor, the electrical resistance of the volume becomes high and the process of charge transportation is dominated by the volume, for any intensity of the applied electric field

Estrutura supramolecular de um derivado de perileno em filmes finos fabricados por evaporação térmica a vácuo

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Estudo estrutural, morfológico, elétrico e óptico de cristais de a-Ag2WO4 dopados com Mo6+ via co-precipitação assistida por PVP

Pós-graduação em Química - IQ