Efeito do Pr2O3 nas propriedades elétricas de varistores à base de SnO2

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)

Caracterização elétrica de blocos varistores à base de SnO2

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)

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.

Nonlinear electrical behaviour of the Cr2O3, ZnO, CoO and Ta2O5-doped SnO2 varistors

The effects of Cr2O3 on the properties of (Zn, Co, Ta)-doped SnO2 varistors were investigated in this study. The samples with different Cr2O3 concentrations were sintered at 1400 degrees C for 2 h. The properties of (Zn, Co, Ta, Cr)-doped SnO2 varistors were evaluated by XRD. dilatornetry, SEM, I-V and impedance spectroscopy. DC electrical characterization showed a dramatic increase ill the breakdown electrical field and in the nonlinear coefficient with the increase in Cr2O3 concentration. The grain size was found to decrease from 13 to 5 mu m with increasing the Cr2O3 content. The impedance data, represented by means of Nyquist diagrams, show two time constants, one at low frequencies and the other at high frequencies. (c) 2005 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

ac conductivity and conduction mechanism of NaNbO3 semiconductor antiferroelectric ceramic: A relaxational approach at high temperature

The electric properties of the sodium niobate perovskite ceramic were investigated by impedance spectroscopy in the frequency range from 5 Hz to 13 MHz and from room temperature up to 1073 K, in a thermal cycle. Both capacitance and conductivity exhibit an anomaly at around 600 K as a function of the temperature and frequency. The electric conductivity as a function of angular frequency sigma(omega) follows the relation sigma(omega)=Aomega(s). The values of the exponent s lie in the range 0.15less than or equal tosless than or equal to0.44. These results were discussed considering the conduction mechanism as being a type of polaron hopping. (C) 2003 American Institute of Physics.

Dependence of La2O3 content on the nonlinear electrical behaviour of ZnO, CoO and Ta2O5 doped SnO2 varistors

The effects of La2O3 on the properties of (Zn, Co, Ta) doped SnO2 varistors were investigated in this study. The samples with different La2O3 concentrations were sintered at 1400 degrees C for 2 h and their properties were characterized by XRD, SEM, I-V and impedance spectroscopy. The grain size was found to decrease from 13 pm to 9 gm with increasing La2O3 content. The addition of rare earth element leads to increase the nonlinear coefficient and the breakdown voltage. The enhancement was expected to arise from the possible segregation of lanthanide ion due to its larger ionic radius to the grain boundaries, thereby modifying its electrical characteristics. Furthermore, the dopants such as La may help in the adsorption of O' to O '' at the grain boundaries characteristics. (c) 2006 Elsevier B.V. All rights reserved.

Structural and electrical properties of ZnO varistors containing different spinel phases

Structural and electrical properties of ZnO varistors were investigated as a function of spinel composition. Six varistor mixtures differing only in chemical composition of spinel, were prepared by mixing separately synthesized constituent phases (DSCP method). Compositions of constituent phases in sintered samples were investigated by changes of lattice parameters of the phases, as well as by EDS analysis of the constituent phases. It was found that compositions of ZnO, intergranular and spinel phases were partially changed during sintering due to redistribution of additives, that was controlled by starting spinel composition and its stability. Electrical characterization showed significant difference in electrical properties of investigated varistors: nonlinearity coefficients ranging from 22 to 55 and leakage currents differing by the order of magnitude. Activation energies of conduction were obtained from ac impedance spectroscopy measurements. Calculated values of activation energies were in the range 0.61-1.0 eV confirming difference in defect structure of ZnO grain boundaries in varistors containing different spinel phases. (C) 2001 Elsevier B.V. Ltd and Techna S.r.l. All rights reserved.

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.

Siloxane-polypropyleneoxide hybrid ormolytes: structure-ionic conductivity relationships

Siloxane-polypropyleneoxide (PPO) hybrids doped with sodium perchlorate (NaClO4) obtained by the sol-gel process were prepared with two PPO molecular weights (2000 and 4000 g/mol) and two sodium concentrations such as [O]/[Na] = 4 and 15 (O being the ether-type oxygen of PPO chains). The structure of these hybrids was investigated by Na-23 nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy at the sodium K-edge (1071.8 eV) whereas complex impedance spectroscopy was used to determine their ionic conductivity. Three sodium sites were determined by NMR. The conjunction of NMR and X-ray absorption results allows us to identify one site in which Na is in a NaCl structure, a second one in which Na is in contact with perchlorate anions. The third site is attributed to mobile sodium species in interaction with the polymeric chain. The relative proportion of the different sites in the materials determines the ionic conductivity of the materials at room temperature: the largest ionic conductivity is 8.9 x 10(-6) Omega(-1) cm(-1) and is observed on the material with the larger amount (at least 85%) of sites in which sodium interacts with the polymer. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Grain boundary electric characterization of Zn7Sb2O12 semiconducting ceramic: A negative temperature coefficient thermistor

The electrical properties of the grain boundary region of electroceramic sensor temperature based on inverse spinel Zn7Sb2O12 were investigated at high temperature. The zinc antimoniate was synthesized by a chemical route based on the modified Pechini method. The electric properties of Zn7Sb2O12 were investigated by impedance spectroscopy in the frequency range from 5 Hz to 13 MHz and from 250 up to 600 degreesC. The grain boundary conductivity follows the Arrhenius law, with two linear branches of different slopes. These branches exhibit activation energies with very similar values; the low-temperature (less than or equal to350 degreesC) and high-temperature (greater than or equal to400 degreesC) regions are equal to 1.15 and 1.16 eV, respectively. Dissimilar behavior is observed on the relaxation time (tau) curve as a function of temperature, where a single slope is identified. The negative temperature coefficient parameters and nature of the polarization phenomenon of the grain boundary are discussed. (C) 2003 American Institute of Physics.

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.

Nanostructured films employed as sensing units in an electronic tongue system

Nanostructured films of lignin (macromolecule extracted from sugar cane bagasse), polypyrrole (conducting polymer) and bis butylimido perylene (organic dye) were used in the detection of trace levels of fluorine (from H2SiF6), chlorine (from NaCIO), Pb+2, Cu+2, and Cd+2 in aqueous solutions. Langmuir monolayers on ultrapure water were characterised by surface pressure-mean molecular area (II-A) isotherms. Langmuir-Blodgett (LB) films were transferred onto gold interdigitated electrodes and used as individual sensing units of an electronic tongue system. Impedance spectroscopy measurements were taken with the sensor immersed into aqueous solutions containing the ions described above in different molar concentrations. Fourier transform infrared absorption (FTIR) was employed to identify possible interactions between the LB films and the analytes in solution, and no significant changes could be observed in the FTIR spectra of BuPTCD and Ppy. Therefore, the results for lignin point to an interaction involving the electronic cloud of the phenyl groups with the metallic ions.

The influence of aluminium on the overall properties of SnO2-based varistors

The influence of aluminium on the development of the microstructure and on the electrical behaviour of the SnO2 center dot Co3O4 center dot Nb2O5 typical varistor system was studied. Two sources of Al were used, alumina (Al2O3) and boehmite (AlO(OH)). The microstructural features were characterised with scanning (SEM) and transmission (TEM) electron microscopies. The different phases present in the studied samples were also studied with XRD, EDS and electron diffraction patterns of selected areas (SAED). Particles containing Sri, Co, Al, and O were unveiled with TEM. Impedance spectroscopy measurements and current density versus electric field characteristics revealed superior electrical properties for samples with AlO(OH). The higher non-linearity (alpha = 19) was achieved with the addition of 0.1% mol of boehmite. The influence of the secondary phases on the electrical properties is also addressed in this work.