Effect of La2O3 doping on the microstructure and electrical properties of a SnO2-based varistor

The effect of La2O3 addition on the densification and electrical properties of the (0.9895 - x) SnO2 + 0.01 CoO + 0.0005 Nb2O5 + x La2O5 system, where x = 0.0005 or 0.00075, was considered in this study. The samples were sintered at 1300 degreesC for 2 and 4 h and a single SnO2 phase was identified by X-ray diffraction. Microstructure analysis by scanning electron microscopy showed that the affect of La2O3 addition is to decrease the SnO2 grain size. J versus E curves indicated that the system exhibits a varistor behavior and the effect of La2O3 is to increase both the non-linear coefficient (alpha) and the breakdown voltage (E-2). Considering the Schottky thermionic emission model the potential height and the width were estimated. The addition of small amounts of La2O3 to the basic system increases the potential barrier height and decreases both grain size and potential barrier width. (C) 2001 Kluwer Academic Publishers.

Effect of oxidizing and reducing atmospheres on the electrical properties of dense SnO2-based varistors

The SnO2 based varistor systems recently presented in the literature appear to have a promising potential in commercial applications. Experimental evidence shows that there is a dependence of nonlinear constant values with thermal treatment under different atmospheres. Thermal treatments in oxygen and nitrogen rich atmospheres at 900 degreesC prove this dependence, indicating that the nonlinear constant values are significantly lower when the material is submitted to a nitrogen atmosphere. Moreover, electrical properties can be restored when the varistor is subjected to thermal treatment at the same temperature in an oxygen atmosphere, indicating that the mechanism seems to be reversible. This paper discusses this behavior focusing in the grain boundary region. Ta2O5 mol% concentrations are also analyzed and the results indicate an optimum Ta2O5 concentration of 0.05 mol% for the electrical properties (alpha = 44 and E-B = 6150 V cm(-1)). (C) 2001 Elsevier B.V. Ltd. 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.

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.

Structural and electrical properties of SrBi2Nb2O9 thin films prepared by chemical aqueous solution at low temperature

SrBi2Nb2O9 (SBN) thin films were prepared by the polymeric precursors method and deposited by dip coating onto Pt/Ti/SiO2/Si(100) substrates. The dip-coated films were specular and crack-free and crystallized during firing at 700 degrees C. Microstructure and morphological evaluation were followed by grazing incident X-ray diffraction (GIXRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The films exhibited somewhat porous grain structure with rounded grains of about 100 nm. For the electrical measurements, gold electrodes of 300 mu m in diameter were sputter deposited on the top surface, forming a metal-ferroelectric-metal (MFM) configuration. The remanent polarization (P-r) and coercive field (E-c) were 5.6 mu C/cm(2) and 100 kV/cm, respectively. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Electrical properties of screen printed BaTiO3 thick films

The deposition of thick film pastes by screen-printing is a relatively simple and convenient method to produce thicker layers with thickness up to 100 mum. In the present work, the barium titanate thick films were prepared from mechanically activated powders based on BaC03 and TiO2. After mixing, the powders were calcined at low temperature by slow heating and cooling rates. The thick films were deposited on to Al2O3 substrates through hybrid technology. The obtained films were fired at 850 degreesC together with electrode material (silver/palladium). The electrical properties of thick films: dielectric permittivity, dielectric losses, Curie temperature, hysteresis loop were reported. The obtained BT thick films can be applied in as multilayer capacitors or in gas sensor application. (C) 2003 Elsevier Ltd. All rights reserved.

Influence of the solution pH on the morphological, structural and electrical properties of Bi3.50La0.50Ti3O12 thin films obtained by the polymeric precursor method

Lanthanum-doped Bi4Ti3O12 thin films (BLT) were deposited on Pt/Ti/SiO2/Si substrates using a polymeric precursor solution. The spin-coated films were specular, crack-free and crystalline after annealing at 700 degrees C for 2 h. Crystallinity and morphological evaluation were examined by X ray diffraction (YRD) and atomic force microscopy (AFM). The stability of the formed complex is of extreme importance for the formation of the perovskite phase. Films obtained from acid pH solution present elongated grains around 200 ran in size, whereas films obtained from basic solution present a dense microstructure with spherical grains (100 nm). The dielectric and ferroelectric properties of the BLT films are strongly affected by the solution pH. The hysteresis loops are fully saturated with a remnant polarization and coercive voltage of P-r=20.2 mu C/cm(2) and V-c = 1.35 V and P-r= 15 mu C/cm(2) and V-c = 1.69 V for the films obtained from basic and acid solutions, respectively. (C) 2005 Elsevier B.V. All rights reserved.

Effect of low contents of a polyaniline derivative on the crystallization and electrical properties of blends with PVDF

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

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.

Investigation of the electrical properties of SnO2 varistor system using impedance spectroscopy

The electrical properties of tin oxide varistors doped with CoO, Nb2O5 and Cr2O3, were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to 400 degrees C. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of O-2 species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at 200 degrees C for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of Cr-Sn at the surface, which promotes the adsorption of the O' and O species which are in turn proposed as being responsible for the barrier formation. (C) 1998 American Institute of Physics. [S0021-8979(98)04719-7]

Effect of Ta2O5 doping on the electrical properties of 0.99SnO(2)center dot 0.01CoO ceramic

The effect of Ta2O5 doping in 0.99SnO(2). 0.01CoO on the microstructure and electrical properties of this ceramic were analyzed in this study. The grain size was found to decrease from 6.87 mu m to 5.68 mu m when the Ta2O5 concentration increased from 0.050 to 0.075 mol%. DC electrical characterization showed a dramatic increase in the current loss and decrease in the non-linear coefficient with the increase of the Ta2O5 concentration. The conduction mechanism is by thermionic emission and the potential barriers are of Schottky type, separated by a thin film. (C) 2000 Kluwer Academic Publishers.

Effect of Fe2O3 doping on the electrical properties of a SnO2 based varistor

The effect of Fe2O3 addition on the densification and electrical properties of the (0.9895 - x) SnO2 + 0.01CoO + 0.005Nb(2)O(5) + xFe(2)O(3) system, where x = 0.005 or 0.01, was considered in this study. The samples were sintered at 1300degreesC for 2 h. Microstructure analysis by scanning electron microscopy showed that the effect of Fe2O3 addition is to decrease the SnO2 grain size. J x E curves indicated that the system exhibit a varistor behavior and the effect of Fe2O3 is to increase both, the non-linear coefficient (alpha) and the breakdown voltage (E-r). Considering the Schottky thermionic emission model the potential height and width were estimated. Small amount addition of Fe2O3 to the basic system increases both the potential barrier height and width. (C) 2002 Kluwer Academic Publishers.

Effect of the heat flux direction on electrical properties of SrBi2Nb2O9 thin films crystallized using a microwave oven

Ferroelectric SrBi2Nb2O9 (SBN) thin films were prepared by the polymeric precursors method and deposited by spin coating onto Pt/Ti/SiO2/Si substrate and crystallized using a domestic microwave oven. It was studied the influence of the heat flux direction and the duration of the thermal treatment on the films crystallization. An element with high dielectric loss, a SiC susceptor, was used to absorb the microwave energy and transfers the heat to the film. Influence of the susceptor position to the sample crystallization was verified, the susceptor was, placed or below the substrate or above the film. The SBN perovskite phase was observed after a thermal treatment at 700 degreesC for 10 min when the susceptor was placed below the substrate and for 30 min when the susceptor was placed above the film. Electrical measurements revealed that the film crystallized at 700 degreesC for 10 min, with the susceptor placed below the film, presented dielectric constant, dielectric loss, remanent polarization and coercive field of, 67, 0.011, 4.2 muC/cm(2) and 27.5 kV/cm, respectively. When the films were crystallized at 700 degreesC for 30 min, with the susceptor placed above the film, the dielectric constant was 115 and the dissipation factor was around of 0.033, remanent polarization and coercive field were 10.8 muC/cm(2) and 170 kV/cm, respectively. (C) 2003 Elsevier B.V. All rights reserved.

The role of oxygen vacancies on the microstructure development and on the electrical properties of SnO2-based varistors

Variations on the microstructure development and on the electrical properties of SnO2-based varistors are discussed on the basis of the oxygen vacancies created or annihilated by the presence of different additives. Electron paramagnetic resonance (EPR) analysis of sintered samples evidenced a substantial increase in the paramagnetic oxygen vacancies concentration when Nb2O5 is added to the SnO2 center dot Co3O4 system. on the other hand, the observed diminution in the concentration of such species after the addition of Fe2O3 indicates solid solution formation. The quantification of paramagnetic oxygen vacancies allowed to confirm the proposed substitutions taking place in the lattice during sintering. These findings are supported by scanning electron microscopy, by density measurements and by current density versus electric field curves. The characterization of secondary phases through EDS assisted SEM and TEM is also reported in this work.

Structure and electrical properties of potassium-doped siloxane-poly(oxypropylene) ormolytes

The structure and the ionic conduction properties of siloxane-poly(oxypropylene) (PPO) hybrids doped with different potassium salts (KCF3SO3, KI, KClO4 and KNO2) are reported for two polymer molecular weights (300 and 4000 g/mol), labelled PPO300 and PPO4000, respectively. The doping concentration, related to the concentration of the ether type oxygen of the PPO chain, is the same whatever the salt and verifies [O]/[K] = 20. Ionic room temperature conductivity shows the highest value for the KCF3SO3 doped PPO4000 hybrid (4 x 10(-7)Omega(-1).cm(-1)). The structure of these hybrids was investigated by X-ray powder diffraction (XRPD) and X-ray absorption spectroscopy (EXAFS and XANES) at the potassium K-edge (3607 eV). XRPD results show that the hybrid matrix is always amorphous and the formation of secondary potassium phases is observed for all the samples, except for the KCF3SO3 doped PPO4000 hybrid. EXAFS results evidence a good correlation between the ionic conductivity and the presence of oxygen atoms as first neighbours around potassium.