Multi-functional properties of CaCu3Ti4O12 thin films

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

Electrical and microstructural properties of microwave sintered SnO2-based varistors

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Grain size effect on the electrical response of SnO2 thin and thick film gas sensors

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

Asymmetrical penetration of microwave in a conducting media and determination of microwave conductivity for very thin samples using electron spin resonance

Dyson's theory of conduction electron spin resonance (CESR) has been used in the limit d less than or equal to delta (d being the thickness of the sample and delta the skin depth of the microwave field) to obtain the microwave conductivity from the (A/B) ratio of the CESR absorbed power derivative. In this work we calculate the CESR absorbed power derivative using Kaplan's approach and show that the (A/B) ratio can be enhanced if asymmetrical penetration of microwave is used, which means that the microwave field enters into the sample from one of the faces. Therefore, the determination of the microwave conductivity from the (A/B) ratio of the CESR line can be performed for thinner samples. Experimentally, asymmetrical penetration can be obtained if one of the sample's faces is covered with a thin gold layer. The determination of microwave conductivity in conducting polymers films is among the possible applications of this method. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

EFFECT OF CR2O3 ON THE ELECTRICAL-PROPERTIES OF MULTICOMPONENT ZNO VARISTORS AT THE PREBREAKDOWN REGION

The effect of Cr2O3 on the electrical properties of the multicomponent ZnO varistors was investigated using voltage-current curves and impedance spectroscopy. The structure and morphological modifications were analysed by X-ray diffraction and scanning electron microscopy, respectively. It was observed in samples with addition 0.1 mol% Cr2O3 that there was an improvement in the electrical properties of the varistors, but the increase in concentration had a deleterious effect on the potential barrier at the grain boundary. The excess Cr2O3 segregates at the grain-boundary region and increases the donor concentration, leading to a higher leakage current.

Study of the thermomechanical and electrical properties of conducting composites containing natural rubber and carbon black

This work describes the preparation and characterization of composite materials obtained by the combination of natural rubber (NR) and carbon black (CB) in different percentages, aiming to improve their mechanical properties, processability, and electrical conductivity, aiming future applications as transducer in pressure sensors. The composites NR/CB were characterized through optical microscopy (OM), DC conductivity, thermal analysis using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMA), thermogravimetry (TGA), and stress-strain test. The electrical conductivity varied between 10(-9) and 10 S m(-1), depending on the percentage of CB in the composite. Furthermore, a linear (and reversible) dependence of the conductivity on the applied pressure between 0 and 1.6 MPa was observed for the sample with containing 80 wt % of NR and 20% of CB. (C) 2007 Wiley Periodicals, Inc.

BARRIER VOLTAGE DEFORMATION OF ZNO VARISTORS BY CURRENT PULSE

The phenomenon of electrical degradation in ZnO varistors was studied by application of high-intensity current pulses. A wave shape of 8 X 20-mu-s and rectangular waves of 1 and 2 ms were used. The degradation was estimated by reference electric-field variation and by Schottky voltage barrier deformation. The results showed that current pulses reduce both the height and the width of the barrier voltage. It was also observed that the donor density N(d) did not change but the surface states density N(s) decreased with degradation.

Electrical characterization of lead zirconate titanate prepared by organic solution route

Two series of lead zirconate titanate (PZT) ceramics with composition Zr/Ti approximate to 53/47 have been prepared by the organic solution route. The effects on the electrical properties of calcination temperature in one series and of sintering time period in the other were examined. Dielectric constant, electrical conductivity and impedance spectroscopy results differed from one series to the other, probably due to differences in structure of the precursor powders, as seen by X-ray diffraction. Tetragonal and rhombohedral phases predominate in the powders used, respectively, in the calcined and sintered series. Physical and electrical behavior of ceramics prepared from predominantly rhombohedral powder suggests the evaporation of PbO. The presence of two semi-circles in impedance plots leads to the association of the low frequency semi-circle to the presence of PbO, which, apparently, was not eliminated from ceramics prepared from predominantly tetragonal powder. (C) 2001 Elsevier B.V. Ltd and Techna S.r.l. All rights reserved.

Electrical and micro structural properties of (Zn, Nb, Fe)-doped SnO2 varistor systems

The electrical and microstructural properties of SnO2-based varistors with the addition of 0.025 and 0.050 mol% of Fe2O3 have been characterised. Electric field (E) versus current density (J) curves showed that the effect of Fe2O3 addition is to increase both the non-linear coefficient and the breakdown voltage. Variations in the potential barrier height were inferred from impedance spectroscopy (IS) analysis. Through transmission electron microscopy (TEM), the presence of precipitates of secondary phases was confirmed. Samples with precipitates displayed poor electrical properties. (c) 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Use of impedance spectroscopy in the study of the effect of Cr concentration on the electrical properties of SnO2-based ceramics

This paper reports a study of influence of Cr concentration on the electrical properties and microstructure of SnO2-based powders doped with Mn and Nb, prepared by an organic route (Pechini method). All the samples were compacted into discs and sintered at 1300 degrees C for 3h, resulting in ceramics with relative density varying between 78% and 98%. The powders were characterized by X-ray diffraction analysis. Impedance spectroscopy characterization indicated that the conductivity decreases as Cr concentration increases, probably due to Cr segregation at grain boundaries, which reduces grain size, increasing the number of resistive boundaries.

Doping of polyaniline by corona discharge

In the present article it is shown that a corona discharge can be employed to dope thin films of polyaniline (PANI) coated on poly(ethylene terephthalate) films, allowing the electrical conductivity to be tuned within the range 10(-10) to 0.3 S cm(-1). A study of the effect of different corona conditions, namely corona treatment for positive and negative polarities, air humidity, treatment time, corona current, and the geometry of the corona triode, on the electrical conductivity of the polyaniline is presented. The results indicate that the corona discharge leads to protonic doping of polyaniline similar to that which occurs in conventional protonic acid solution doping. Atomic force microscopic analysis shows that, as the PANI is exposed to the corona discharge, its globular morphology is disrupted leading to the appearance of droplet-like features and a significant decrease in the average height and surface roughness. Doping by corona discharge presents several advantages over the conventional solution method namely that it is a dry process which does not require use of chemicals reagents, and which is both rapid and avoids dopant migration. The latter can be important for applications of PANI in microelectronic devices. (C) 2000 American Institute of Physics. [S0021-8979(00)01608-X].

CURRENT ASPECTS OF FUEL ETHANOL-PRODUCTION IN BRAZIL

Brazil has become a great producer of bioethanol using sugarcane as the basic raw material. Fed-batch process and continuous process are used. Biogas generation from vinasse, production of dry yeast, and autolyzed bagasse for animal feed are making the ethanol production less polluting and more profitable. Bagasse surplus has also been converted into electrical energy. Another alternative use for bioethanol is its conversion to petrochemical derivatives. Up to the present, however, this conversion has been carried out on only a small scale by the industry.

Effect of combined bending strain and thermal cycling on the voltage-current characteristic curves of Bi-2223 tapes

High critical temperature superconductors are evolving from a scientific research subject into large-scale application devices. In order to meet this development demand they must withstand high current capacity under mechanical loads arising from thermal contraction during cooling from room temperature down to operating temperature (usually 77 K) and due to the electromagnetic forces generated by the current and the induced magnetic field. Among the HTS materials, the Bi2Sr2Ca2Cu3Ox, compound imbedded in an Ag/AgMg sheath has shown the best results in terms of critical current at 77 K and tolerance against mechanical strain. Aiming to evaluate the influence of thermal stress induced by a number of thermal shock cycles we have evaluated the V-I characteristic curves of samples mounted onto semicircular holders with different curvature radius (9.75 to 44.5 mm). The most deformed sample (epsilon = 1.08%) showed the largest reduction of critical current (40%) compared to the undeformed sample and the highest sensitivity to thermal stress (I-c/I-c0 = 0.5). The V-I characteristic curves were also fitted by a potential curve displaying n-exponents varying from 20 down to 10 between the initial and last thermal shock cycle.

Oriented growth of Bi3.25La0.75Ti3O12 thin films on RuO2/SiO2/Si substrates by using the polymeric precursor method: Structural, microstructural and electrical properties

c-axis oriented Bi3.25La0.75Ti3O12 (BLT) thin films were grown on a RuO2 top electrode deposited on a (100) SiO2/Si substrate by the polymeric precursor method. X-ray diffraction and atomic force microscope investigations indicate that the films exhibit a dense, well crystallized microstructure having random orientations with a rather smooth surface morphology. The electrical properties of preferred oriented Bi3.25La0.75Ti3O12 (BLT) thin films deposited on RuO2 bottom electrode leaded to a large remnant polarization (P-r ) of 17.2 mu C/cm(2) and (V-c ) of 1.8 V, fatigue free characteristics up to 10(10) switching cycles and a current density of 2.2 mu A/cm(2) at 5 V. We found that the polarization loss is insignificant with nine write/read voltages at a waiting time of 10,000 s. Independently of the applied electric field the retained switchable polarization approached a nearly steady-state value after a retention time of 10 s.

THEORY OF NON-STEADY-STATE ELECTRICAL CHARACTERISTICS OF METAL-INSULATOR-METAL STRUCTURES WITH SCHOTTKY BARRIERS AND UNIFORMLY DISTRIBUTED INTERFACE IMPURITY STATES

The metal-insulator (or amorphous semiconductor) blocking contact is still not well understood. In the present paper, we discuss the non steady state characteristics of Metal-lnsulator-Metal Structure with non-intimate blocking contacts (i.e. Metal-Oxide-Insulator-Metal Structure). We consider a uniform distribution (in energy) of impurity states in addition to impurity states at a single energy level within the depletion region. We discuss thermal as well as isothermal characteristics and present expressions for the temperature of maximum current (T-m) and a method to calculate the density of uniformly distributed impurity states. The variation of mobility with electrical field has also been considered. Finally we plot the theoretical curves under different conditions. The present results are closing into available experimental results.