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.

Influence of Ta2O5 on the electrical properties of ZnO- and CoO-doped SnO2 varistors

SnO2-based varistors doped with 0.5% cobalt, 0.5% zinc and various tantalum amounts were prepared by the solid-state route. Experimental evidence shows that small quantities of Ta2O5 improve the nonlinear properties of the samples significantly. It was found that samples doped with 0.05 mol% Ta2O5 exhibit the highest density (98.5%), the lowest electric breakdown field (E-b = 1050 V/cm) and the highest coefficient of nonlinearity (alpha = 11.5). The effect of Ta2O5 dopant could be explained by the substitution of Ta5+ by Sn4+. (C) 2004 Elsevier Ltd and Techna S.r.l. All rights reserved.

Electrical properties of YSZ/NiO composites prepared by a liquid mixture technique

We report the preparation and characterization of yttria-stabilized zirconia/nickel oxide composites (YSZ/NiO). This composite is the precursor material of the cermet YSZ/Ni, which is used as solid oxide fuel cell anode material. The performance of the anode is strongly dependent on the microstructural properties of the cermet. Therefore, the control of the microstructure of the YSZ/NiO composite is a key step for the fabrication of high-performance anodes. In this study, the composites were prepared by a modified liquid mixture technique. Scanning electron microscopy analysis evidenced the good dispersion of the phases and that NiO nanoparticles are spread over the YSZ surface. Sintered pellets were studied by X-ray diffraction and impedance spectroscopy. The main results show that the composite is comprised of a well-dispersed mixture of the two phases. The electrical conductivity data show that there is a strong dependence of the transport mechanism on the relative composition of phases. (c) 2005 Elsevier Ltd. All rights reserved.

ANISOTROPY OF ELECTRICAL-PROPERTIES IN ALPHA-FE2O3 CERAMICS

Electrical conductivity and thermoelectric power measurements carried out in a heamatite ceramic showed a strong anisotropy in directions normal and parallel to the uniaxial pressing direction. This behaviour is similar to that verified in alpha-Fe2O3 single crystal. The results suggest that the extended structural defects, generated during sintering, disturb the magnetic order on the (001) planes of alpha-Fe2O3 and limit the mobility of n type carriers.

Influence of the LiNbO3 and KNbO3 seeds on the sintering and electrical properties of PMN ceramic

The effect of LiNbO3 and KNbO3 seeds on the microstructure and dielectric characteristics of PMN ceramic prepared by columbite route have been investigated with the addition of 0, 1, and 2-wt% of seeds. X-ray diffraction, Scanning Electron Microscopy and an impedance analyzer were used to characterize the influence of seeds on physical characteristics and dielectric properties of PMN. LiNbO3 -seeded PMN samples present a significant increase in the amount of perovskite phase. The addition of LiNbO3 seeds in sintered PMN ceramics at 1100degreesC during 4 h causes a decrease in the porosity and the amount of pyrochlore phase. Weight losses during sintering of PMN ceramics are suppressed more significantly for LiNbO3 -seeded PMN. T-m of PMN ceramics changes with seeds concentration. KNbO3 seeds displace T-m to lower temperature whereas LiNbO3 causes its elevation. Dielectric constants of approximately 13,000 at 1 kHz was measured at -5degreesC in PMN ceramics with 1-wt% of LiNbO3 seeds.

Influence of temperature on the microstructure and electrical properties of BBT thin films

The BBT films were prepared by a spin-coating process from the polymeric precursor method (Pechini process). In order to study the influence of the temperature on the BBT microstructure and electrical properties, the films were deposited on platinum coated silicon substrates and annealed from 700degreesC to 800degreesC for 2 hours in oxygen atmosphere. The crystallinity of the films was examined by X-ray diffraction while the surface morphology was analysed by atomic force microscope. The dielectric properties and dissipation factor of BaBi2Ta2O9 films at 1 MHz were observed. The polarization-electric field hysteresis loops revealed the ferroelectric characteristics of BaBi2Ta2O9 thin films.

Low temperature synthesis and electrical properties of PbTiO3 thin films prepared by the polymeric precursor method

Ferroelectric PbTiO3 thin films were successfully prepared on a Pt(111)Ti/SiO2/Si(100) substrate for the first time by spin coating, using the polymeric precursor method. X-ray diffraction patterns of the films indicate that they are polycrystalline in nature. This method allows low temperature (500 degrees C) synthesis and high electrical properties. The multilayer PbTiO3 thin films were granular in structure with a grain size of approximately 110-120 nm. A 380-nm-thick film was obtained by carrying out four cycles of the spin-coating/heating process. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses showed the surface of these thin films to be smooth, dense and crack-free with low surface roughness (=3.4 nm). At room temperature and at a frequency of 100 kHz, the dielectric constant and the dissipation factor were, respectively, 570 and 0.016. The C-V characteristics of perovskite thin film prepared at low temperature show normal ferrolectric behavior. The remanent polarization and coercive field for the films deposited were 13.62 mu C/cm(2) and 121.43 kV/cm, respectively. The high electrical property values are attributed to the excellent microstrutural quality and chemical homogeneity of thin films obtained by the polymeric precursor method. (C) 2000 Elsevier B.V. S.A. All rights reserved.

Investigation of electrical properties of tantalum doped SnO2 varistor system

Ta2O5 doped SnO2 varistor systems containing 0.5 mol% ZnO and 0.5 mol% Coo were prepared by mixed oxide method. Considering that ZnO and Coo oxides are densification additives only the SnO(2)center dot ZnO center dot CoO ceramics cannot exhibit electrical nonlinearity. A small amount of Ta2O5 improves the nonlinear properties of the samples greatly. The height and width of the defect barriers were calculated. It was found that samples doped with 0.05 mol% Ta2O5 exhibit the highest density (98.5%), the lowest electric breakdown field (E-b = 1100 V/cm) and the highest coefficient of nonlinearity (alpha = 11.5). The effect of Ta2O5 dopant could be explained by the substitution of Ta5+ by Sn4+. A grain-boundary defect barrier model for the SnO(2)center dot ZnO center dot CoO center dot Ta2O5 varistor system was also introduced. (c) 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Microstructure and electrical properties of perovskite (Pb, La)TiO3 thin film deposited at low temperature by the polymeric precursor method

High-quality (Pb, La)TiO3 ferroelectric thin films were successfully prepared on a Pt(111)/Ti/SiO2/Si(100) substrate for the first time by spin coating, using the polymeric precursor method. The X-ray diffraction patterns show that the films are polycrystalline in nature. This method allows for low temperature (500 degrees C) synthesis, a high quality microstructure and superior dielectric properties. The effects on the microstructure and electrical properties were studied by changing the La content. The films annealed at 500 degreesC have a single perovskite phase with only a tetragonal or pseudocubic structure. As the La content is increased, the dielectric constant of PLT thin films increases from 570 up to 1138 at room temperature. The C-V and P-E characteristics of perovskite thin films prepared at a low temperature show normal ferroelectric behavior, representing the ferroelectric switching property. The remanent polarization and coercive field of the films deposited decreased due to the transformation from the ferroelectric to the paraelectric phase with an increased La content. (C) 2001 Kluwer Academic Publishers.

Temperature dependence on the electrical properties of Ba(Ti0.90Zr0.10)O-3:2V ceramics

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Effect of atmosphere on the electrical properties of TiO2-SnO2 varistor systems

This work illustrates the advancement of research on TiO2-based electroceramics. In this work will be presented that the addition of different dopants, as well as thermal treatments at oxidizing and inert atmosphere, influences of the densification, the mean grain size and the electrical properties of the TiO2-based varistor ceramics. Dopants like Ta2O5, Nb2O5, and Cr2O3 have an especial role in the barrier formation at the grain boundary in the TiO2 varistors, increasing the nonlinear coefficient and decreasing the breakdown electric field. The influence of Cr'(Ti) is to increase the O' and O'(2) adsorption at the grain boundary interface and to promote a decrease in the conductivity by donating electrons to O-2 adsorbed at the grain boundary. In this paper, TiO2 and (Sn,Ti)O-2-based studies of polycrystalline ceramics, which show a non-linear I-V electrical response typical of low voltage varistor systems are also presented. All these systems are potentially promising for varistor applications. (C) 2004 Kluwer Academic Publishers.

Electrical Properties of Textured Niobium-Doped Bismuth Titanate Ceramics

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