Influence of TiO2 and AI(2)O(3) on the electrical and microstructural properties of SnO2 ceramics obtained by the Pechini method

Tin oxide has wakened up great scientific and technological interest for its potential use in varistors production and as gas sensor. In order to improve the microstructural and electrical properties in SnO2 varistor ceramics, the influence of differents dopants used, like TiO2 and Al2O3, is under research. The effect of TiO2 and Al2O3 on the properties of Sn-Co-Nb varistor systems obtained by the Pechini method has been investigated in this work. Characterization of synthesized raw material was performed by X-Ray Diffraction (XRD) and Scanning Electronic Microscopy (SEM). The microstructural and electrical characterization of sintered samples show that the TiO2 favors the grain growth and the Al2O3 contributes to the decrease it, effect that is manifested in the Sn-Co-Nb varistor systems. Breakdown field increase up to 6300V/cm with increasing Al2O3 content and non-linear coefficients with alpha=22 were obtained.

Optical emission and electron capture of rare-earth trivalent ions located at distinct sites in SnO(2) thin films

We present photoluminescence and decay of photo excited conductivity data for sol-gel SnO(2) thin films doped with rare earth ions Eu(3+) and Er(3+), a material with nanoscopic crystallites. Photoluminescence spectra are obtained under excitation with several monochromatic light sources, such as Kr(+) and Ar(+) lasers, Xe lamp plus a selective monochromator with UV grating, and the fourth harmonic of a Nd: YAG laser (4.65eV), which assures band-to-band transition and energy transfer to the ion located at matrix sites, substitutional to Sn(4+). The luminescence structure is rather different depending on the location of the rare-earth doping, at lattice symmetric sites or segregated at grain boundary layer, where it is placed in asymmetric sites. The decay of photo-excited conductivity also shows different trapping rate depending on the rare-earth concentration. For Er-doped films, above the saturation limit, the evaluated capture energy is higher than for films with concentration below the limit, in good agreement with the different behaviour obtained from luminescence data. For Eu-doped films, the difference between capture energy and grain boundary barrier is not so evident, even though the luminescence spectra are rather distinct.

Formation of SnO2 gels from dispersed sols in aqueous colloidal solutions

Transparent SnO2 gels were obtained from SnCl4 aqueous solution. The sol formation from tin oxihydroxy peptization in different concentrations and by electrolyte addition in solution was measured. It was verified that the residual presence of chloride ions compromises the colloidal system stability. The sol-gel transition was investigated as a function of the quantity of solid particles in the aqueous environment and of aging time at 60°C by infrared spectroscopy and rheological measurements. The transition from plastic to pseudoplastic flow observed with the increase in loading suggests that a continuous and three-dimensional network formation is closely related to hydrogen bridges and/or hydrogen clusters, culminating in the gel formation. © 1990.

Effect of electrolyte on the gelation and aggregation of SnO2 colloidal suspensions

Electrolytes may modify the physical-chemical characteristics of colloidal particle interfaces in suspension, which can favour gel or aggregate formation. The influence of NH4Cl loading on the aggregation and gelation of SnO2 colloidal suspensions was investigated using measurements of rheology, turbidity and infrared spectra. A rapid aggregate growth for samples with Cl- > 20 mM was observed. With increasing age, gelation was observed due to formation of interaggregate bonds. For concentration of Cl- between 20 and 9 mM, the aggregation process was slower allowing the formation of gel with a network which was not destroyed as the gel was submitted to a small rate of shear. As aging continues, the condensation reaction between OH groups gave rise to the formation of Sn-O bonds, irrespective of the electrolyte loading. © 1992 Elsevier Science Publishers B.V. All rights reserved.

Drying of SnO2 hydrogels: effect of the electrolyte

The effect of the electrolyte (NH+ 4, Cl-) on the drying of SnO2 hydrogels was investigated by linear shrinkage, mass loss, gravimetric thermal analysis and infrared spectroscopy. Results show that the drying mechanism for monolithic SnO2 gels is highly dependent on the concentration of the electrolyte solution inside the pores. For higher concentrations, the drying process is governed by capillary forces while for the smaller ones (≤20 mM) syneresis shrinkage becomes predominant just before the end of the first drying period. This phenomenon is related to condensation reaction among the superficial OH groups and may hamper formation of monolithic SnO2. © 1992 Elsevier Science Publishers B.V. All rights reserved.

Spectroscopic characterization of SnO2 gels - Code: CP11

Excitation and dynamic emission spectra of Eu3+ ions were simultaneously used with FTIR and Raman spectroscopy to study the structural evolution during SnO2 sol → gel → xerogel conversion. Results make evident an increase of the surroundings symmetry for the Eu3+ ions dissolved in SnO2 matrix and a decrease of the amount of hydroxo groups (Sn-OH) during drying. These phenomena were associated to the pursuit of the condensation reaction after gelation. © 1994 Kluwer Academic Publishers.

Short Range Order Evolution in the Preparation of SnO2 Based Materials

The evolution of Eu3+ doped SnO2 xerogels to the cassiterite structure observed during sintering was studied by means of Eu3+ spectroscopy, XRD and EXAFS at the Sn K-edge. Eu3+ ions adsorbed at the surface of colloidal particles present a broad distribution of sites, typical of oxide glasses. With sintering at 300°C, this distribution is still broadened. Crystallization is clearly observed by the three techniques with increasing sintering temperature. It is found that the addition of Eu3+ limits the crystallite growth.

Electrical and optical characteristics of SnO2 thin films prepared by dip coating from aqueous colloidal suspensions

Starting from aqueous colloidal suspensions, undoped and Nb5+ doped SnO2 thin films have been prepared by using the dip-coating sol gel process. X-ray diffraction results show that films are polycrystalline with crystallites of average size1-4nm. Decreasing the thickness of the films and increasing the Nb5+ concentration limits the crystallite size growth during firing. Complex impedance measurements reveal capacitive and resistive effects between adjacent crystallites or grains, characteristic of electrical potential barriers. The transfer of charge throughout these barriers determines the macroscopic electrical resistance of the layer. The analysis of the optical absorption spectra shows that the samples present more than 80% of their transmittance in the visible region and the value of the band gap energy increases with decreasing crystallite size. © 1997 Chapman & Hall.

EXAFS and XRD Study of the Structural Evolution during Isothermal Sintering of SnO2 Xerogels

The formation of an ordered (crystalline) phase during isothermal sintering of SnO2 monolithic xerogels, at 200, 250, 300, 400, 500, 600 and 700°C, has been analyzed by the combined use of EXAFS and XRD techniques. For the desiccated gel (110°C), EXAFS results show the formation of small microcrystallites with the incipient cassiterite structure. Between 110 and 250°C, the dehydratation reaction leads to an amorphization evidenced by a decrease of the long and short range crystallographic order. It is due to fissure formation in the xerogel network. For higher temperatures, a continuous coagulation of the crystallites occurs, leading to grain growth. Grain and pore growth obeys the same kinetic relation, so that the microstructure grows by simple enlargement while its morphology is static.

Influence of viscoelastic characteristics of the colloidal suspensions on the formation of SnO2 supported membranes

SnO2 supported membranes have been prepared by sol-casting on alumina tubular substrate, using aqueous colloidal suspensions prepared by sol-gel route. The viscoelastic behaviour during sol ageing was analyzed by dinamic rheologial measurements. The complex viscosity and the storage and loss moduli have been followed during the sol-gel transition and the results have been correlated with the linear aggregates growth and the scalar percolation models. The scanning electron microscopy has evidenced that the homogeneity and thickness of the membrane depend on the sol ageing time. Crack-free and homogeneous membranes have been obtained for coated layers of 0.5μm thickness. © 1997 Trans Tech Publications.

Electrical properties of the SnO2-based varistor

The non-linear electrical properties of CoO-doped and Nb205-doped SnO2 ceramics were characterized. X-ray diffraction and scanning electron microscopy indicated that the system is single phase. The electrical conduction mechanism for low applied electrical field was associated with thermionic emission of the Schottky type. An atomic defect model based on the Schottky double-barrier formation was proposed to explain the origin of the potential barrier at the ceramic grain boundaries. These defects create depletion layers at grain boundaries, favouring electron tunnelling at high values of applied electrical field. © 1998 Chapman & Hall.

Effect of atmosphere and dopants on sintering of SnO2

Tin oxide is an n-type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of a non-isovalent oxide doping The sintering and grain growth of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnO2 powder the surface state depends only on the interaction of atmosphere molecules with the SnO2 surface. Inert atmosphere like argon or helium promotes oxygen vacancy formation at the surface due to reduction of SnO2 to SnO at the surface and liberation of oxygen molecules forming oxygen vacancies. As consequence surface diffusion is enhanced leading to grain coarsening but no densification. Oxygen atmosphere inhibits the SnO2 reduction decreasing the surface oxygen vacancy concentration. Addition of dopants with lower valence at sintering temperature creates extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide.

Photodesorption and electron trapping in n-type SnO2 thin films grown by dip-coating technique

Thin films of undoped and Sb-doped (2 atg%) SnO2 have been prepared by sol-gel dip-coating technique on borosilicate glasses. Variation of photoconductivity excitation with wavelength and optical absorption indicate indirect bandgap transition with energy of ≅ 3.5 eV. Conductance as function of temperature indicates two levels of capture with 39 and 81 meV as activation energies, which may be related to an Sb donor and oxygen vacancy respectively. Electron trapping by these levels are practically destroyed by UV photoexcitation (305 nm) and heating in vacuum to 200°C. Gas analysis using a mass spectrometer indicates an oxygen related level, which may not be desorbed in the simpler O2 form.

Plant Cell Wall Research Related to Evolution and Chemical Defenses

The plant cell wall is composed mainly of polysaccharides some constituted of repeating units of a single sugar, as cellulose or by two or more sugars grouped in repeating oligosaccharide blocks as the galactomannans and xyloglucans. Variations in composition and fine structure of these cell wall polysaccharides have been used as taxonomic markers and in the comprehension of the evolutive process, particularly in the Leguminosae. Partial hydrolysis of these compounds give rise to oligomers, some of which are capable of eliciting the synthesis of defensive substances in plants named phytoalexins. Species which differ in respect to phytoalexin liberation also differ in cell wall composition, particularly in the pectic fraction of the wall. Pectinases (mainly endopolygalacturonases) present in fungi, have been shown to hydrolyze plant cell walls yielding phytoalexin-eliciting oligosaccharides which differ in composition and in eliciting capacity in different species. These differences can be associated with the capacity of a given species to produce phytoalexins. On the other hand, the phytoalexin induction in plants is being used as a method of producing novel bioactive secondary metabolites.