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.

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.

Effects of synthesis and processing on supersaturated rare earth-doped nanometric SnO2 powders

This paper presents and discusses some of the results of the effects of processing on rare earth-doped nanosize SnO2. Several relevant factors that may influence the characteristics of the final product are studied. The influence of two preparation routes and two heat-treatment conditions on the incorporation of dopants is investigated. The route whereby a soluble salt is used as the dopant source is found to provide the highest degree of dopant incorporation, even under the least favorable heat-treatment conditions.

Electrochemical oxidation of methanol on Pt nanoparticles dispersed on RuO2

Pt-modified RuO2 was prepared by a sol-gel procedure on titanium substrates in the form of thin films of similar to2-mum thickness. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that these films actually consist of Pt nanoparticles dispersed in RuO2 and that neither metallic Ru nor Pt-Ru alloy are present on the surface. Electrodes with different Pt:Ru nominal compositions were prepared and their electrocatalytic activity for the oxidation of methanol was investigated by potential sweeps and chronoamperometry. The results obtained show an enhancement effect for methanol oxidation that can be interpreted as associated to the formation of hydrous oxides on the RuO2 surface.

Ultraviolet excitation of photoconductivity in thin films of sol-gel SnO2

Tin dioxide (SnO2) thin film photoconductivity spectra were measured for a large temperature range using a deuterium source, the intensity of photocurrent spectra in the range 200-400 nm is temperature dependent, and the photocurrent increases in the ultraviolet even for illumination with photon energies much higher than the bandgap transition. This behavior is related to recombination of photogenerated electron-hole pairs with oxygen adsorbed at grain boundaries, which is consistent with nanoscopic crystallite size of sol-gel deposited films. (c) 2005 Elsevier Ltd. All rights reserved.

Nonlinear properties and stability of SnO2 varistors prepared by evaporation and decomposition of suspensions

SnO2 varistors doped with CoO, Cr2O3 and Nb2O5 were prepared by evaporation and decomposition of suspensions. The composition of the varistors was optimized to improve electrical properties, such as nonlinearity, leakage current and electrical stability. The best results were achieved with the following composition: 99.15% SnO2 +0.75% CoO+0.05% Cr2O3 +0.05% Nb2O5. Samples showed high density, reaching 99.5% of the theoretical density, as well as an homogeneous microstructure. The nonlinear coefficient was higher than 30 in the current range from 10(-7) to 10(-2) A/cm(2). The leakage current was 0.86 mu A/cm(2). These samples showed high stability of electrical parameters when they were exposed to high current of 27 mA/cm(2) for different time periods up to 30 min. (c) 2005 Elsevier Ltd. 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.

The influence of yttrium on a typical SnO2 varistor system: Microstructural and electrical features

The influence of yttrium oxide, Y2O3, on the microstructure development of the SnO(2)center dot Co(3)O(4)center dot Nb2O5 typical varistor system was studied with scanning (SEM) and transmission (TEM) electron microscopies. The different phases present in the studied samples were characterized through XRD, EDS and selected area diffraction patterns (SAD). Particles of Co2SnO4 were observed with TEM in every sample, whereas clusters of the pyrochlore phase T2Sn2O7 were observed with SEM in samples with 0.05, 0.10 and 0.25 mol% of Y2O3. The higher non-linearity (a = 16) was achieved with the addition of 0.05 mol% of Y2O3. The influence of the secondary phases on the electrical properties is also addressed in this work. (c) 2005 Published by Elsevier B.V.

Microstructural evolution during sintering of CoO doped SnO2 ceramics

Addition of 0.5 mol% of CoO into SnO2 promotes densification of this oxide to 99% of the theoretical density during sintering. TEM in this system reveals that after sintering at 1210 degrees C a secondary phase of Co2SnO4 is precipitated at the SnO2 grain boundaries during cooling. This phase is formed by diffusion of Co ions from the bulk to the grain boundary during sintering leaving needle-like defects at the grain bulk. The high resolution TEM micrograph of this system sintered at 1210 degrees C and 1400 degrees C showed an amorphous grain boundary region low in cobalt, indicating that the Co2SnO4 phase is precipitated from this region. (C) 1999 Elsevier B.V. Limited and Techna S.r.l. All rights reserved.

Catalysis and temperature dependence on the formation of ZnO nanoparticles and of zinc acetate derivatives prepared by the sol-gel route

The chemical and structural nature of powders prepared from the zinc acetate-derived precursor using the sol-gel route is discussed. The influence of the synthesis temperature and of the hydrolytic catalyst on the structural features of the powder is focused on the basis of X-ray powder diffraction (XRPD) and extended X-ray absorption fine structure (EXAFS) measurements and complemented with density and thermoanalysis (TG-DTA) results. EXAFS and XRPD results show that no-washed nanoparticulate powders are composed of a mixture of ZnO (wurtzite), zinc acetate, and zinc hydroxyacetate. The latter has a layered structure typical of hydroxy double salts (HDS). The main component of no-washed powders is always unreacted zinc acetate solid but the relative amount of the zinc-based compounds depends on the nature of the hydrolytic catalyst, hydrolysis ratio, and of synthesis temperature. According to the proportion of the three zinc-based compounds, three families of powders could be distinguished. The amount of ZnO nanoparticles (1.6 +/- 0.6 nm) decreases as the synthesis temperature increases, as the hydrolysis ratio decreases, or by changing from basic to acid catalysis. This finding suggests that the formation of zinc compounds is controlled by the equilibrium between hydrolysis-condensation and complexation-reprecipitation reactions.

Electro-optical properties of Er-doped SnO2 thin films

Photoconductivity of SnO2 sol-gel films is excited, at low temperature, by using a 266 nm line-fourth harmonic-of a Nd:YAG laser. This line has above bandgap energy and promotes generation of electron-hole pairs, which recombines with oxygen adsorbed at grain boundary. The conductivity increases up to 40 times. After removing the illumination on an undoped SnO2 film, the conductivity remains unchanged, as long as the temperature is kept constant. Adsorbed oxygen ions recombine with photogenerated holes and are continuously evacuated from the system, leaving a net concentration of free electrons into the material, responsible for the increase in the conductivity. For Er doped SnO2, the excitation of conductivity by the laser line has similar behavior, however after removing illumination, the conductivity decreases with exponential-like decay. (C) 2003 Elsevier Ltd. All rights reserved.

Densification and coarsening of SnO2-based materials containing manganese oxide

The present work presents results on natural sintering of tin dioxide ceramics, prepared by a chemical route or by conventional mixing and containing manganese (X-Mn = Mn/(Mn + Sn)(atomic) with 0 less than or equal to X(Mn)less than or equal to 0.15). This cation, which is practically insoluble in SnO2 network, stays at the grain surface. During thermal treatment (500 degrees C less than or equal to T-s less than or equal to 1400 degrees C), as long as the manganese surface concentration is lower than a critical value, equal to 5.10(-6) mol m(-2), no densification takes place. As soon as this value is reached, densification and grain growth occur simultaneously. The shrinkage kinetics is fast and high rho/rho(t) values can be obtained (for example. rho/rho(t)=0.95 for T-s=1300 degrees C and X-Mn=0.004). The dependence between manganese content, manganese distribution, grain size and sintering behaviour is also discussed. (C) 1998 Published by Elsevier B.V. Limited.