Characterization of varistors system s SnO2-MnO2 based.

This work focussed on the effect of addition of Nb2O5 on the electrical and microestructural properties of SnO2-MnO2 based ceramics. It was also studied the effect of heat-treatments in a oxygen-rich atmosphere in these properties. The parent system was 99,5% SnO2-0,5% MnO2 in molar ratio, a dense system that did not present non-ohmic behavior. The addition of Nb2O5 on this system promoted an increase on the charge carrier number in the SnO2 matrix and, then, varistor properties were observed. Otherwise, the upper doping level, 0.25% mol of Nb2O5 system showed an higher porosity, due to the decrease on the thermal sintering properties, that depleted the varistor characteristic of this system. A detailed study on the electrical properties were performed using impedance spectroscopy and the obtained results showed that Nb2O5 tends to form an electronic trapping level between the valence and the conduction bands, modifying the potential barrier properties formed in the grain boundaries region. In order to explain the obtained results, it was proposed a parallel equivalent circuit model, based on the grain boundaries contributions to electrical properties. Heat-treatments under oxygen-rich atmosphere did not changed the observed electrical properties in the systems, due to the presence of numerous secondary phases (rich in oxygen and manganese), localized in triple points in the grain boundaries. It is pointed the there is a oxygen local enrichment in these phases, not significantly altering the observed macroscopical electrical characteristics in the analyzed systems.Universidade Federal de Minas GeraisNeste trabalho estudou-se o efeito da adição de Nb2O5 nas propriedades elétricas e microestruturais de cerâmicas a base de SnO2-MnO2. Foi também estudado o efeito do tratamento térmico em atmosfera oxidante sobre essas propriedades. O sistema de partida foi 99,5% SnO2-0,5% MnO2 em mol, que é um sistema denso mas não apresenta propriedades não-ohmicas. A adição de Nb2O5 ao sistema proporciona um aumento no número de portadores de carga na matriz do SnO2 e o sistema passa a ter propriedades varistoras. Entretanto, o sistema contendo 0,25% em mol de Nb2O5 apresentou alta porosidade, devido à diminuição nas propriedades de sinterização do sistema com o aumento da concentração de Nb2O5, o que impossibilita esse sistema de ser utilizado como varistor. Um estudo detalhado das propriedades elétricas foi feito por espectroscopia de impedância e mostra que o Nb2O5 forma um nível de aprisionamento de elétrons entre a banda de valência e a banda de condução que modifica as propriedades da barreira de potencial formada na região do contorno de grão. Para explicar os resultados, é proposto um modelo de circuito equivalente em paralelo baseado apenas nas contribuições do contorno de grão para as propriedades elétricas. O tratamento térmico em atmosfera oxidante não alterou as propriedades elétricas dos sistemas devido a uma grande quantidade de precipitados (ricos em oxigênio e manganês) localizados em pontos triplos entre grãos. É proposto que haja um enriquecimento local com espécies de oxigênio adsorvidas nesses precipitados, mas que não alteram as propriedades elétricas macroscópicas dos sistemas estudados

Influence of processing parameters on nanomaterials synthesis efficiency by a carbothermal reduction process

In this work, the influence of synthesis parameters on the synthesis efficiency of tin oxide nanomaterials was studied by using the carbothermal reduction method in a sealed tube furnace. The parameters were the starting material, temperature and time of synthesis as well as the gas flux. The starting material was tin dioxide mixed with carbon black in a molar proportion of 1.5:1 and 1:1. The temperature range was from 950 to 1,125 A degrees C with a step of 25 A degrees C, and the synthesis times used were 15, 30, 45, 60, 75, 90, and 120 min. Using optimum values of the above parameters, the gas flux was changed to verify its influence. After completion of the syntheses, we found a grayish-black material inside the tube which was characterized by X-ray diffraction and scanning electron microscopy. The results showed that the collected material is composed of nanobelts (with width around 60 nm) and disks that grew preferentially in the SnO phase. A model based on the oxide vapor pressure was proposed to evaluate the efficiency of the process, and the results showed good agreement between experimental data and the proposed model. Based on the results obtained, the best conditions to obtain a homogeneous material with 95% efficiency is using a starting material in the molar proportion Sn:C of 1.5:1, a temperature of 1,132 A degrees C for 75 min, and a N(2) gas flux of 80 sccm.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Study of ITO@PMMA composites by transmission electron microscopy

ITO nanowires were synthesized by carbothermal reduction process, using a co-evaporation method, and have controlled size, shape, and chemical composition. The electrical measurements of nanowires showed they have a resistance of about 102 Ω. In order to produce nanocomposites films, nanowires were dispersed in toluene using an ultrasonic cleaner, so the PMMA polymer was added, and the system was kept under agitation up to obtain a clear suspension. The PMMA polymer was filled with 1, 2, 5 and 10% in weight of nanowires, and the films were done by tape casting. The results showed that the electrical resistance of nanocomposites changed by over 7 orders of magnitude by increasing the amount of filler, and using 5 wt% of filler the composite resistance decreased from 1010 Ω to about 104 Ω, which means that percolation threshold of wires occurred at this concentration. This is an interesting result once for nanocomposites filled with ITO nanoparticles is necessary about 18% in weight to obtain percolation. The addition of filler up to 10 wt% decreased the resistance of the composite to 103 Ω, which is a value close to the resistance of wires. The composites were also analyzed by transmission electron microscopy (TEM), and the TEM results are in agreement with the electrical ones about percolation of nanowires. These results are promising once indicates that is possible to produce conductive and transparent in the visible range films by the addition of ITO nanowires in a polymeric matrix using a simple route. © 2011 Materials Research Society

Influence of thermal annealing treatment in oxygen atmosphere on grain boundary chemistry and non-ohmic properties of SnO2 center dot MnO polycrystalline semiconductors

The present work studied the influence of thermal treatment in oxygen rich atmosphere on heterogenous junctions in Mn-doped SnO2 polycrystalline system presenting varistor behavior. The samples were prepared by conventional oxide mixture methodology, and were submitted to heat treatment in oxygen rich atmosphere at 900 degrees C for 2h. The samples were characterized by X-ray diffraction, scanning electron microscopy, dc and ac electrical measurements. The results showed that there is an evident relationship between the microstructure heterogeneity and non-ohmic electrical properties. It was found that for this SnO2 center dot MnO-based varistor system the heat treatment in oxygen rich atmosphere does not necessarily increase the varistors properties, which was related to the decrease in the grain boundary resistance. The results are compared with Co-doped SnO2 varistors and ZnO based varistors. (C) 2008 WILEY-VCH Verlay GmbH & Co. KGaA, Weinheim

Schottky-type grain boundaries in CCTO ceramics

In this work we studied electrical barriers existing at CaCu(3)Ti(4)O(12) (CCTO) ceramics using dc electrical measurements. CCTO pellets were produced by solid state reaction method and X-ray diffractograms showed which single phase polycrystalline samples were obtained. The samples were electrically characterized by dc and ac measurements as a function of temperature, and semiconductor theory was applied to analyze the barrier at grain boundaries. The ac results showed the sample's permittivity is almost constant (10(4)) as function of temperature at low frequencies and it changes from 100 to 10(4) as the temperature increases at high frequencies. Using dc measurements as a function of temperature, the behavior of barriers was studied in detail. Comparison between Schottky and Poole-Frenkel models was performed, and results prove that CCTO barriers are more influenced by temperature than by electric field (Schottky barriers). Besides, the behavior of barrier width as function of temperature was also studied and experimental results confirm the theoretical assumptions. (C) 2011 Elsevier Ltd. All rights reserved.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Efficient microwave-assisted hydrothermal synthesis of CuO sea urchin-like architectures via a mesoscale self-assembly

For the first time, we provide evidence for both heterogeneous nucleation at the mesoscopic scale and the progression of the non-classical crystallization process during the microwave synthesis of cupric oxide (CuO) with sea urchin-like morphology. This work sheds light on the general mechanism of the growth of CuO architectures.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Assessment of the superior photocatalytic properties of Sn2+-containing SnO2 microrods on the photodegradation of methyl orange

Abstract A microporous Sn2+-containing SnO2 material presenting microrod morphology and a surface area of 93.0 m2 g–1 was synthesized via a simple hydrothermal route. Sn2+ ions were detected in the interior of the material (15.8 at.%) after the corrosion of a sample through sputtering. The material’s optical properties have demonstrated the absorption of a considerable fraction of visible light up to wavelengths of 671 nm, due to the presence of Sn2+ states in the material’s band structure. The analysis of the internal crystalline structure of a single microrod was carried out with the aid of a focused ion beam microscope and indicated that the material is mesocrystalline down to nanoscale level. It was proposed that the Sn2+ ions occupy intergranular sites in the highly defective crystalline structure of the material and that Sn2+ states, as well as its relatively large surface area, are responsible for the material’s superior photoactivity. The synthesized material was tested as a photocatalyst to decompose hazardous contaminants in water. The photocatalytic performance of the material was much higher than those of commercial TiO2 and SnO2 materials, decomposing nearly all methyl orange (an azo-dye model) content in water (10 mg L–1) in 6 min under UV irradiation for a photocatalyst dose of 5.33 g L–1. The photodegradation of methyl orange was also verified under visible light